Difference between revisions of "Theory of ladder escapes"

Latest revision as of 18:14, 19 October 2024

The object of this page is to formalise precisely what it means for a pattern to be a ladder escape. To do this, we first formalise what it means to be a ladder.

Informally, a ladder escape (say, a 4th row ladder escape) is supposed to give the attacker a guarantee that their 4th row ladder will be able to connect to the edge, no matter how far away from the ladder escape the ladder starts. So strictly speaking, to check that a pattern is a 4th row ladder escape, we must check that the attacker can connect to the edge from an infinite set of positions. This raises the issue of how one can check in a finite time whether a given pattern is a 4th row ladder escape.

This issue is resolved on this page for 2nd, 3rd, 4th, and 5th row ladders. It might be possible to resolve it for 6th row ladders but this has not yet been done, partly because such ladders are of little practical use. For 7th row ladders we run into a new difficulty – Blue can simply ignore the ladder and play near the escape, because no appropriate 6th row edge template seems to be known which will connect an ignored 7th row ladder to the edge. This presents a theoretical obstruction which is currently unresolved. It may in theory be that there are no 7th row ladders at all.

For the purpose of our analysis, we assume that all ladders move from left to right along the red bottom edge, with Red being the attacker. Of course, the analogous analysis also applies to ladders moving in the opposite direction or along different edges.

The analysis of 2nd–4th row ladders on this page was originally contributed by the user Wccanard in 2016.

A note on terminology. The usual definition of a template is a pattern that has a stated property (for example, being connected) and is also minimal with that property. In other words, a template is usually defined by two properties: validity and minimality. For the purpose of this page, we are mostly concerned with validity. Since it would be awkward to write "template but not necessarily minimal" throughout all of the definitions and proofs on this page, we adopt the convention, on this page only, that "template" means a pattern that is valid but not necessarily minimal. We will then speak of a "minimal template" when necessary.

1 Algebraic notation
2 Second row ladders
3 Third row ladders
4 Fourth row ladders
5 Fifth row ladders
6 Sixth row ladders and up
7 Second-to-fourth row switchbacks
8 Third-to-fifth row switchbacks
9 Second and fourth row parallel ladders
10 Third and fifth row parallel ladders
11 Second and fourth row terraced ladders

Algebraic notation

Before we start, let us introduce some notation that will be useful.

Open patterns

A pattern is a set of cells, each of which may be empty or occupied by a stone of either color. In this article, we will only be concerned with patterns that include a red board edge. A pattern is open on the left if it comes with some cells marked "+" on its left side. No cells to the left of those "+"s may be part of the pattern. A pattern is open on the right if it comes with some cells marked "−" on its right side. No cells to the right of those "−"s may be part of the pattern. A pattern is open on both sides if it is open on the left and on the right. A pattern is closed if it is not open on either side. For example, the following four patterns are open on the right, open on both sides, open on the left, and closed, respectively:

The cells labelled "+" (if any) are called the left boundary of the pattern, the cells labelled "−" are called its right boundary, and the carrier of a pattern consists of all cells that are part of the pattern (empty or not), except the boundaries.

Addition

Suppose P is a pattern that is open on the right, Q is a pattern that is open on the left, and the right boundary of P has the same number of cells and shape as the left boundary of Q. Then we write P+Q for the pattern obtained by joining P and Q along their common boundary. More specifically, P+Q is obtained as follows: delete the right boundary from P and the left boundary from Q. Then glue the patterns P and Q together along the line where the boundaries were deleted from each. For example:

+

=

It is important to note that the carrier of P+Q consists of just the carriers of P and Q, without the boundary cells that have been deleted. The purpose of the boundary cells "+" and "−" is just to indicate where the patterns will be attached. It is possible to add more than two patterns, for example:

+

=

Note that addition is only well-defined if P and Q can be glued together without their carriers overlapping. We will be careful to ensure that this is always the case. However, the addition is associative, i.e., (P + Q) + R is well-defined if and only if P + (Q + R) is well-defined, and in that case, they are equal.

The shift operation

If P is a pattern that is open on the left, we write 1 + P for the pattern obtained from P by shifting its left boundary one column to the left, and adding a column of empty cells where the boundary used to be. More generally, for any integer n ≥ 0, we write n + P for iterating this operation n times, i.e., for shifting the left boundary of P to the left by n columns and filling the additional space with empty cells. For example:

1 +

=

4 +

=

Note that empty cells are only added to the height of the boundary. For a pattern that is open on the right, we can do exactly the same thing on the other side, i.e., P + n is obtained by shifting the right boundary to the right by n columns, filling the additional space with empty cells. For example:

+ 1 =

+ 4 =

We call this the shift operation. Note that it is associative: If P and Q have matching boundaries, then (P + n) + Q = P + (n + Q).

The reduce operation

If P is a pattern that is open on the left, we write ↑ + P for the pattern obtained from P by erasing the topmost "+" cell from its left boundary. The cell that formerly contained the "+" is no longer part of the pattern (i.e., it is not replaced by an empty cell). For example:

↑ +

=

We call this the reduce operation. The shift and reduce operations can be combined with each other and with addition of patterns. For instance, n + ↑ + m + P is the pattern obtained from P by first shifting its left boundary by m cells to the left, then reducing the size of that boundary by one cell, and then shifting it by another n cells to the left. For example:

2 + ↑ + 3 +

=

Second row ladders

Definition of ladder

There is no issue at all with defining a 2nd row ladder. Informally, a 2nd row ladder looks like this:

Note that at each point in the ladder, Blue's move is forced. Red can choose to continue pushing the ladder as long as she wants to. We formally define a second row ladder as follows:

Definition. A second row ladder is a pattern like this:

L2:

Here, the red stone is on the second row, and we call it the ladder stone. Red's goal is to connect the ladder stone to the bottom edge. The cell immediately below and to the right of the ladder stone is empty. We denote this pattern by L2.

Definition of ladder escape

Before we give the formal definition of a second row ladder escape, let us consider an example. The following pattern is an example of a second row ladder escape.

Of course directly underneath the pattern is the bottom (red) edge. The cells marked "+" indicate where the ladder connects. The reason this is a second row ladder escape is that however far away the ladder is,

Red can guarantee a connection from the ladder stone (marked 1) to the bottom edge.

Let us clarify what the hexes marked "+" in the ladder escape pattern mean. They indicate the last point where the 2nd row ladder is allowed to start. So for example, saying that the pattern above is a second row ladder escape means (among other things) that Red must win the following position:

Here, Red's ladder stone is marked "1", and the claim (easily verified) is that even with Blue to play, Red can connect the ladder stone to the bottom:

The reason that the pattern is a second row ladder escape is that this escape sequence works even if the ladder is a long way away:

Even here, Red can force a connection to the edge, even if it is Blue's move, because Blue must keep defending on the first row and Red keeps attacking on the second row,

and now we are back at the previous position with the ladder right next to the escape, where we have already seen that Red can break through to the edge. We can now give a more formal definition of a second row ladder escape.

Definition. A second row ladder escape template (or simply second row ladder escape) is given by the following data. It is a pattern P that is open on the left (see algebraic notation above), with a boundary of this shape:

subject to the following axiom: for all n ≥ 0, the position L2 + n + P is a virtual connection from the ladder stone (marked 1) to the edge.

Concretely, this means that any position consisting of a second row ladder L2,

followed directly to the right by an arbitrary number (zero or more) of pairs of vacant hexes on the first and second rows,

followed by the second row ladder escape pattern (where the ladder slots into the escape by putting the ladder or rightmost column of vacant hexes onto the hexes marked "+"), allows Red to connect the ladder stone to the edge.

Terminology and notation: If we have a left-open pattern whose boundary is of the correct shape, but we are not sure whether it satisfies the axiom of a second row ladder escape, then we refer to it as a candidate for a second row ladder escape (or simply candidate if the rest is clear from the context). A candidate is valid if it is actually a ladder escape.

We also define what it means for a ladder escape template to be minimal.

Definition. A 2nd row ladder escape template is minimal if the following two things are true. First, removing any hex from the pattern, or removing a red stone from the pattern (and replacing it with an empty hex) gives a new pattern which is not a 2nd row ladder escape template any more. And second, if the two hexes directly to the right of the two cells marked "+" are both vacant hexes in the pattern, then moving the cells marked "+" one hex to the right results in a new pattern which is not a 2nd row ladder escape.

Below, we will use analogous terminology and notations for ladders and ladder escapes on the 3rd and higher rows.

Characterization of ladder escapes

The definition of a 2nd row ladder escape allows the ladder to be an arbitrary distance away from the escape, which is of course what we want in practice; there is no reason that the escape should be right next to the ladder. However, this means that we cannot directly use the definition to check that something is a 2nd row ladder escape, because this would require checking that infinitely many patterns are virtual connections. Can we find some finite criterion for checking 2nd row ladder escapes? Fortunately, as every Hex player knows, the answer is yes. We have the following theorem:

Theorem 1. Consider a candidate P for a 2nd row ladder escape. Schematically:

(Here the asterisks indicate the carrier of P, which can contain any stones at all, and can be of any shape or size, as long as it includes no cells to the left of the cells marked "+"). Then P is a valid 2nd row ladder escape if and only if L2+P is a virtual connection for Red.

Proof. If the ladder escape is valid, then by definition, L2+n+P is a virtual connection for all n ≥ 0, and in particular for n = 0. This proves the left-to-right implication.

To go the other way we actually have to play some Hex, but it's pretty trivial. We must show that L2+n+P is a virtual connection for all n. This is an easy induction on n. For n = 0, the claim is true by assumption. If n > 0, then Blue must play directly below Red's ladder stone (or else Red will connect to the edge immediately), and now Red can play a ladder stone at distance n−1 on the second row, which is a virtual connection by induction hypothesis. □

Examples

We can use Theorem 1 to prove that all of the following patterns are minimal second row ladder escapes. Most of these templates are taken from David King's website, and there are several more there. For several of the templates, the corresponding pattern on David King's site is not minimal by our definition; for these templates, we have moved the cells marked "+" to the right to make the template minimal.

The shaded cell is not part of the template, and can be occupied by Blue.

In the below templates, the stone marked "↓" indicates a stone connected to the bottom edge, but the connection is not shown. The connection from 10 to the edge must not use any of the empty hexes in the pattern.

Third row ladders

Definition of ladder

There is a minor issue with defining ladders on the 3rd and higher rows. We want a definition that is useful in practice and not too restrictive. For example, we surely want this to be a third row ladder:

even though there are a few blue stones on the first row. It is intuitively clear (and also provably true) that these blue stones cannot be of any help to Blue (they can never play a crucial role in any blue connection). So although we want a 3rd row ladder to have no stones on the first three rows to the right of the ladder (until we reach the escape), we do not want to also guarantee that there are no stones on the first row to the left of the ladder. We formally define third row ladders as follows.

Definition. A third row ladder is a pattern like this:

L3:

The red stone is again called the ladder stone, and Red's goal is to connect the ladder stone to the bottom edge. We denote this pattern by L3.

The key part of the definition is that we are guaranteeing the triangle of three empty hexes under the red ladder stone. This is a minimal requirement, because for example if one of these cells were filled,

then in reality the game could look like this,

and Blue can block the ladder with this move.

Definition of ladder escape

We have seen a lot of the formalism of ladder escapes in the above section on second row escapes. However there is a new twist with third row ladder escapes, because Blue can defend against a third row ladder in more than one way: Blue can at some stage decide to yield to the second row. The following definition is unsurprising.

Definition. A third row ladder escape is given by the following data. It is a pattern P that is open on the left, with a boundary of the shape

To be a third row ladder escape, the pattern must satisfy the property that for all n ≥ 0, L3 + n + P is a virtual connection from the Red ladder stone to the bottom edge, with Blue to move.

Like for second row escapes, a pattern that has the required shape for a ladder escape, but it is not (yet) known to be a valid ladder escape, is called a candidate.

In pictures, for the pattern

(where the carrier is schematically indicated by stars) to be a 3rd row ladder escape, it must give rise to a virtual connection when we attach a 3rd row ladder at distance 0,

or at distance 1,

or at distance 6,

or at any other distance.

Characterization of ladder escapes

Just like for second row ladder escapes, we again find ourselves in the situation that trying to use the definition to check that something is a 3rd row ladder escape involves checking that infinitely many positions are virtual connections. Once again, we have a theorem that allows us to replace this by a finite condition.

Theorem 2. Consider a candidate P for a 3rd row ladder escape. Assume that (a) L2+↑+P is a virtual connection and (b) L3+P is a virtual connection, each from the ladder stone to the bottom edge. Then P is a valid third row ladder escape.

Proof. First note that by Theorem 1, because L2+↑+P is a virtual connection, P escapes all 2nd row ladders. Now under the assumptions of the theorem, we must show that L3+n+P is a virtual connection for all n ≥ 0. We prove this by induction on n. For n = 0, the claim is true by assumption (b). Now suppose the claim is true for some n. To show the claim for n+1, consider the position L3+n+1+P. The first three columns of this position look like this:

(This is followed by n more columns of three empty hexes and by the pattern P). Blue has three possible moves in a triangle under stone 1, and Blue needs to play one of these or he will lose instantly. We analyze all three moves in turn.

For the first, Red pushes the ladder and will connect to the edge because by induction hypothesis, L3+n+P connects to the edge, so stone 3 connects to the edge, and so stone 1 does too.

For the second, Red just wins outright, i.e., we do not need to use the induction hypothesis.

And for the third, Red responds like this. Since stone 3 is a 2nd row ladder stone, it is connected to the edge because, as we noted above, ↑+P is a 2nd row ladder escape. Therefore stone 1 is also connected.

The induction is now complete, showing that P is a 3rd row ladder escape. □

Remark: in more concrete terms, Theorem 2 states that a pattern P is a 3rd row ladder escape if the pattern becomes a virtual connection (from the ladder stone to the edge) when we attach each of the following two patterns to its left boundary:

A:

B:

In contrast to the situation with 2nd row ladders, while Theorem 2 is sufficient to show that a position is a 3rd row ladder escape, it is not necessary. For example, consider the following third row ladder escape template P:

One can check directly that L3+2+P and L2+↑+2+P are both virtual connections, so that 2+P is a 3rd row ladder escape by Theorem 2. In particular, L3+n+P is a virtual connection for all n ≥ 2. Moreover, one can check that L3+P and L3+1+P are also virtual connections, so that P is a valid 3rd row ladder escape.

It is, however, not a valid 2nd row ladder escape for ladders at distance 0, because in the position L2+↑+P,

the ladder stone marked "1" cannot connect to the edge.

Theorem 2 is therefore not sufficient to check that a given pattern is a 3rd row ladder escape. We need to work a little harder to get a necessary and sufficient condition for 3rd row ladder escapes.

Lemma 3 (2nd to 3rd row jump). Any 3rd row ladder escape also escapes 2nd row ladders that start at distance 2 or greater. More specifically, if L3+P is a virtual connection, then so is L2+↑+2+P.

The lemma is perhaps easier understood in pictures: given any 3rd row ladder escape, replacing the three cells marked "+"

by the pattern

yields a 2nd row ladder escape.

Proof. Assume L3+P is a virtual connection. We must show that L2+↑+2+P is a virtual connection. It looks as follows, with P added to it:

But Blue must play 2, and Red can jump to 3. Then 3 is a 3rd row ladder stone, and is connected to the edge because L3+P is a virtual connection by assumption. Therefore, 1 is also connected.

□

We now finally get a necessary and sufficient condition for 3rd row ladder escapes in the following theorem.

Theorem 4. Consider a candidate P for a 3rd row ladder escape. Then P is a valid 3rd row ladder escape if and only if L3+P, L3+1+P, and L3+2+P are virtual connections.

Proof. The left-to-right implication is trivial, since by definition, if P is valid then L3+n+P is a virtual connection for all n, including n = 0, 1, 2. For the opposite implication, assume that L3+P, L3+1+P, and L3+2+P are virtual connections. By Lemma 3, L2+↑+2+P is a virtual connection. By Theorem 2 and the assumption about L3+2+P, 2+P is a 3rd row ladder escape. It follows that L3+n+P is a virtual connection for all n ≥ 2. Since we additionally assumed this to be the case for n = 0 and n = 1, P is a valid third row ladder escape. □

As a matter of fact, Theorem 4 is not tight. We can get the following better result. However, the proof of Theorem 4 generalizes more easily to 4th row and higher ladders, which is why it is of interest.

Theorem 5. Consider a candidate P for a 3rd row ladder escape. Then P is a valid 3rd row ladder escape if and only if L3+P and L3+1+P are virtual connections.

Proof. The left-to-right implication is trivial. For the right-to-left implication, by Theorem 4, it suffices to show that L3+2+P is a virtual connection. Indeed, consider Blue's options in the position L3+2+P:

As usual, there are only two possible moves for Blue to avoid losing immediately. If Blue moves at 2, then Red can respond at 3, which connects to the edge because L3+1+P is a virtual connection by assumption.

If Blue instead moves at 2, then Red responds as follows, which connects to the edge because L3+P is a virtual connection by assumption.

□

Examples

Here are some examples of third row ladder escapes. Again most of these are taken from David King's website. For several of the ladder escape templates, the version shown on David King's website is not minimal by our definition; in these cases, we have moved the cells marked "+" to the right to make the template minimal. All of the templates in this section have been proven to be third row ladder escapes using Theorem 5. All of them are minimal. As before, a stone marked "↓" is assumed to be connected to the bottom row, but the connection is not shown. Any shaded cells are not part of the pattern and can be occupied by Blue.

The following third row ladder escapes also escape 2nd row ladders at distance 0 or greater:

The following third row ladder escapes also escape 2nd row ladders at distance 1 or greater (but not at distance 0):

The following third row ladder escapes also escape 2nd row ladders at distance 2 or greater (but not at distance 0 or 1):

The version of this last pattern on David King's website has the cells marked "+" (he uses arrows) sloping in the other direction; the location that is shown here makes the template minimal.

Fourth row ladders

Definition of ladder

Definition. A fourth row ladder is a pattern like this:

L4:

Again, the red stone is called the ladder stone and Red wants to connect the ladder stone to the bottom edge. We denote this pattern by L4.

The key part of the definition is that the 6 hexes forming a triangle below the ladder stone are all vacant. Note that even filling in one of these can invalidate the ladder: even if we fill in the bottom left corner of the triangle,

then Blue has this move,

which is easily seen to stop the ladder. To establish the ladder, Red needs at a minimum those 6 vacant hexes under her ladder stone.

Definition of ladder escape

The definition of a 4th row ladder escape is entirely analogous to that of 2nd and 3rd row ladder escapes.

Definition. A fourth row ladder escape is given by a pattern P that is open on the left with a boundary of this shape.

Moreover, it must satisfy that for all n ≥ 0, L4 + n + P is a virtual connection from the Red ladder stone to the bottom edge.

As before, a candidate is a pattern that has the correct shape, but is not (yet) known to be a valid escape.

Characterization of ladder escapes

We have already encountered all of the relevant ideas. If you have worked through the ideas in the second and third row escapes then this will be relatively easy, other than the actual Hex, which this time is quite fun!

Theorem 6. Consider a candidate P for a 4th row ladder escape. If L2+↑+↑+P, L3+↑+P, L4+P, and L4+1+P are virtual connections, then P is a 4th row ladder escape.

Proof. The idea of the proof is the same as for 3rd row ladders. First observe that by Theorems 1 and 2, since L2+↑+↑+P and L3+↑+P are virtual connections, ↑+P escapes all 3rd row ladders and ↑+↑+P escapes all 2nd row ladders. We must prove that L4+n+P is a virtual connection for all n ≥ 0. We proceed by induction on n. The base cases n = 0 and n = 1 hold by assumption. For the induction step, assume the claim is true for n ≥ 1. We need to prove the claim for n+1.

Consider the position L4+n+1+P. It looks like this, with n−1 additional columns of four vacant hexes and the pattern P attached on the right:

We need to prove that the ladder stone 1 is connected to the edge.

The five moves marked 2 below all lose instantly to Red 3:

The two moves marked 2 below also lose instantly:

The move marked 2 below can be answered by Red 3, moving us to position L4+n+P, which is a virtual connection by the induction hypothesis.

Move 2 below leads us to a 2nd row ladder, which ↑+↑+P escapes, so 5 (and therefore 1) is connected to the edge:

Both moves marked 2 below lead us to a 3rd row ladder, which ↑+P escapes, so 3 (and therefore 1) is connected to the edge:

Move 2 below also leads to a 3rd row ladder (note Blue 4 must be in the triangle left and below from Red 3; Blue can also play out the bridge between 1 and 3 but this doesn't help):

Move 2 below leads us to a 2nd row ladder:

The final choice for move 2 below also gives a 2nd row ladder.

This completes the induction, so P is a 4th row ladder escape. □

Remark: In more concrete terms, Theorem 6 states that P is a 4th row ladder escape if adding each of the following patterns to P gives a virtual connection.

A:

B:

C:

D:

Remark: Theorem 6 is analogous to Theorem 2. It gives a sufficient, but not a necessary condition for a candidate to be a 4th row ladder escape. Once again, the criterion in Theorem 6 can be checked in a finite amount of time. To get a theorem with a necessary and sufficient condition, we need another "jump lemma":

Lemma 7 (3rd to 4th row jump). Any 4th row ladder escape also escapes 3rd row ladders that start at distance 3 or greater. More specifically, if L4+P and L4+1+P are virtual connections, then so is L3+↑+3+P.

Proof. Consider the position L3+↑+3+P, which looks as follows, with P added to it:

There are only two possible moves for Blue that don't lose immediately. If Blue moves at 2, then Red can respond at 3, which is a 4th row ladder stone and connects to the edge because L4+1+P is a virtual connection by assumption.

In Blue moves instead at 2 in the following diagram, then Red can respond as shown.

Now Red's stone 9 is a 4th row ladder stone. Although the additional red stone 5 does not belong in the L4 template, this stone can only help Red. By assumption, L4+P is a virtual connection, and so stone 9, and therefore stone 1, is connected to the edge. □

We then arrive at a necessary and sufficient condition for fourth row ladder escapes.

Theorem 8. Given a candiate P for a 4th row ladder escape. Then P is a valid 4rd row ladder escape if and only if L4+P, L4+1+P, L4+2+P, ..., L4+6+P are virtual connections.

Proof. The proof is similar to that of Theorem 4. Again, the left-to-right implication is trivial. For the right-to-left implication, assume that L4+P, L4+1+P, L4+2+P, ..., L4+6+P are virtual connections. By Lemma 7 applied to P and 2+P, we know that L3+↑+3+P and L3+↑+5+P are virtual connections. By Lemma 3 applied to ↑+3+P, we know that L2+↑+2+↑+3+P is a virtual connection, and therefore also L2+↑+↑+5+P, which differs from L2+↑+2+↑+3+P only in that it contains two additional empty hexes. Since L2+↑+↑+5+P, L3+↑+5+P, L4+(5+P), and L4+(6+P) are virtual connections, we know by Theorem 6 that 5+P is a valid 4th row ladder escape. Therefore, L4+n+P is a virtual connection for all n ≥ 5. Since we assumed this to be also true for n = 0, 1, 2, 3, 4, it follows that P is a valid 4th row ladder escape. □

Remark: Like Theorem 4, it is likely that Theorem 8 is not tight, in the sense that there probably exists an even simpler condition that is necessary and sufficient for 4th row ladder escapes (perhaps analogous to Theorem 5). Also, in practice, Theorem 6 is often easier to check since it involves fewer conditions.

Examples

Here are some examples of fourth row ladder escapes. Most are taken from David King's website. In each case we have moved the column of "+"s as far as possible to the right to yield a minimal template. The validity of all of these escapes has been proved using Theorem 8.

The following fourth row ladder escape templates also escape 2nd and 3rd row ladders at distance 0 or greater:

The following fourth row ladder escape templates also escape 2nd row ladders and 3rd row ladders at distance 1 and greater.

The following fourth row ladder escape templates also escape 2nd row ladders at distance 2 and greater, as well as 3rd row ladders at distance 1 or greater. The stone marked "↓" is assumed to be connected to the bottom edge, although the connection is now shown.

The following fourth row ladder escape templates also escape 2nd row ladders at distance 1 and greater, as well as 3rd row ladders at distance 0 or greater.

The following fourth row ladder escape template also escapes 2nd row ladders at distance 2 and greater, as well as 3rd row ladders at distance 0 or greater.

Fifth row ladders

Definition of ladder

Definition. A fifth row ladder is a pattern like this:

L5:

As usual, the red stone is called the ladder stone and Red's goal is to connect it to the bottom edge. We denote this pattern by L5.

Unlike in the case of 2nd, 3rd, and 4th row ladders, this time it is not sufficient for a triangle of cells below and to the right of the ladder stone to be empty. We also need three additional empty cells to the left of this triangle. This is a minimal requirement; if even one of these cells is occupied by Blue, for example like this:

then Blue can block the ladder with this move:

The main line is complex; see for example this Little Golem discussion thread. Many of the main lines of defense involve Blue playing an upside-down version of Tom's move, for example like this:

Note that Blue's 1 is connected to 5 by double threat at "*", and 7 is Tom's move upside-down, i.e., with the top line of blue stones serving as the "edge". Therefore, to establish the ladder, Red needs at minimum the specified 13 vacant hexes under the ladder stone.

Definition of ladder escape

The definition of a 5th row ladder escape is as expected.

Definition. A fifth row ladder escape is a pattern P that is open on the left with boundary of this shape:

It must satisfy the following axiom: for all n ≥ 0, L5 + n + P connects the red ladder stone to the bottom edge, with Blue to move. As usual, a candiate is such a pattern that satisfies everything except perhaps the axiom.

Characterization of ladder escapes

Theorem 9. Consider a candiate P for a fifth row ladder escape. Assume L5+P, L5+1+P, L5+2+P, L4+↑+P, L4+↑+1+P, L3+↑+↑+P, and L2+↑+↑+↑+P are all virtual connections. Then P is a 5th row ladder escape.

Proof. The proof idea is the same as for 3rd and 4th row ladders, but there are a lot more cases to consider. First, note that by previous theorems, ↑+P escapes all 4th row ladders, ↑+↑+P escapes all 3rd row ladders, and ↑+↑+↑+P escapes all 2nd row ladders. We prove by induction on n that L5+n+P is a virtual connection for all n ≥ 0. The base cases n = 0, 1, 2 are true by assumption. For the induction step, assume the claim is true for n ≥ 2. We need to prove the claim for n+1.

Consider the position L5+n+1+P, which looks like this (followed by an additional n−2 columns of five empty hexes and the pattern P):

The eight moves marked 2 below all lose instantly to Red 3 by edge template IV-1-a:

The three moves marked 2 below also lose instantly by edge template IV-1-a:

The six moves marked 2 below give a 4th row ladder, which ↑+P escapes.

This leaves us with 11 more moves to consider. If Blue pushes the ladder by making the move marked 2 below, Red can answer 3, moving us to position L5+n+P, which is a virtual connection by the induction hypothesis.

Move 2 below gives a 3rd row ladder, which ↑+↑+P escapes.

If Blue plays move 2 below, Red can answer 3, forcing Blue to respond in the ziggurat below and to the left of stone 3. If Blue plays in any of the cells marked 4, Red plays 5 and gets a 4th row ladder, which ↑+P escapes. Blue could have also first intruded upon the bridge between 1 and 3, but this does not help. From now on, we tacitly ignore bridge intrusions that are not helpful to Blue.

If, on the other hand, Blue plays 4 below, then Red gets a 2nd row ladder, which ↑+↑+↑+P escapes.

If Blue plays move 2 below, Red gets a 2nd row ladder, which ↑+↑+↑+P escapes.

If Blue plays move 2 below, Red can answer 3, forcing Blue to respond in one of the hexes marked 4. Then Red can play 5 and gets a 3rd row ladder, which ↑+↑+P escapes.

Similarly, if Blue plays move 2 below, Red can answer 3, forcing Blue to respond in one of the hexes marked 4. Then Red can play 5 and gets a 3rd row ladder, which ↑+↑+P escapes.

If Blue plays move 2 below, Red can answer 3. There are only four hexes where Blue can respond without losing outright. If Blue moves in one of the three hexes marked 4, then Red gets a 3rd row ladder, which ↑+↑+P escapes.

If Blue instead moves in the hex marked 4 below, then the sequence plays out slightly differently, but Red still gets a 3rd row ladder.

If Blue plays move 2 below, Red can answer 3. Then Blue must respond in any of the hexes marked "+", or else Blue will immediately lose to edge template III1b or a ziggurat.

If Blue responds in one of the two hexes marked 4, Red gets a 3rd row ladder.

If Blue instead responds with move 4 below, Red gets a 2nd row ladder.

If Blue instead responds with move 4 below, Red still gets a 2nd row ladder.

If Blue plays move 2 below, Red can answer 3. Then Blue must respond in one of the hexes marked "+" to avoid immediately losing to edge template III1b.

If Blue responds in one of the hexes marked 4 below, Red gets a 3rd row ladder.

If Blue instead responds in one of the hexes marked 4 below, Red gets a 2nd row ladder.

If Blue instead responds at 4, Red gets a 3rd row ladder.

If Blue instead responds at 4, Red gets a 2nd row ladder.

If Blue plays move 2 below, Red can answer 3. Then Blue must respond in one of the hexes marked "+" to avoid immediately losing to edge template III1b or a bridge.

If Blue responds in one of the hexes marked 4 below, Red gets a 2nd row ladder.

If Blue instead responds at 4 below, Red also gets a 2nd row ladder.

Finally, if Blue plays move 2 below, Red gets a 2nd row ladder.

This completes the induction, so P is a 5th row ladder escape. □

Remark: In more concrete terms, Theorem 9 states that P is a 5th row ladder escape if adding each of the following patterns to P gives a virtual connection.

A:

B:

C:

D:

E:

F:

G:

Like Theorems 2 and 5, Theorem 9 gives a sufficient, but not necessary condition for 5th row ladder escapes. We do not currently have a necessary and sufficient condition. One problem is that we have no appropriate "jump lemma" from 4th to 5th row ladders. In fact, we can prove that no such jump lemma is possible.

Lemma 10 (No jumping from 4th to 5th row). Suppose Red is the attacker in a 4th row ladder. Given enough Blue pieces on the 6th row, and enough space on the right, jumping is not an option for Red. If Red tries to jump, Blue can block the ladder, and Red will get at most a 2nd row ladder in the opposite direction.

Proof. If Red tries to jump, Blue can play as follows.

This is exactly an upside-down version of the situation in Theorem 16 below. No matter where Red plays next, Blue can prevent Red from connecting. The hexes marked "*" are not required by Blue (i.e., they could be occupied by Red). Under optimal play, Red gets at most a 2nd row ladder in the opposite direction as follows:

See the proof of Theorem 16 for a detailed discussion of all the possible moves. □

Lemma 10 is a significant obstacle to establishing a necessary and sufficient criterion for 5th row ladder escapes. We do have the following generalization of Theorem 9, which gives a weaker sufficient condition (it is perhaps also necessary, but this has not been shown):

Theorem 11. Given a candiate P for a 5th row ladder escape. If there is some n ≥ 0 such that L5+P, L5+1+P, ..., L5+n+P, L5+n+1+P, L5+n+2+P, as well as L4+↑+n+P, L4+↑+n+1+P, L3+↑+↑+n+P and L2+↑+↑+↑+n+P, are virtual connections, then P is a valid 5th row ladder escape.

Proof. This follows directly from Theorem 9 applied to n+P. □

Examples

Here are some examples of fifth row ladder escapes. The validity of these escapes has been proved using Theorem 11. These escapes are minimal.

The following fifth row ladder escapes also escape 2nd to 4th row ladders at distance 0 or greater:

The following fifth row ladder escape also escapes 2nd row ladders at distance 1 or greater, and 3rd and 4th row ladders at distance 0 or greater:

The following fifth row ladder escape also escapes 2nd row ladders at distance 2 or greater, 3rd row ladders at distance 0 or greater, and 4th row ladders at distance 1 or greater:

The following fifth row ladder escape also escapes 2nd row ladders at distance 2 or greater, 3rd row ladders at distance 0 or greater, and 4th row ladders at distance 2 or greater:

Sixth row ladders and up

Because of the 2nd-to-6th row switchback trick, 6th and higher row ladders do not exist in the usual sense. More specifically, even if we allow an arbitrary amount of empty space under the ladder stone, it is not possible for the attacker to keep pushing the ladder. Consider the following situation:

Let's assume there is an arbitrary amount of empty space in the bottom 4 rows to the left of this diagram. The stone marked "1" is connected to the top, and looks like it could be the ladder stone for a potential 6th row ladder. If such a ladder were possible, the red stones on the M-file should certainly escape it.

From Blue's point of view, Blue is the attacker in an upside-down 2nd row ladder. Blue can therefore use an upside-down version of the 2nd-to-6th row switchback trick. To do so, Blue plays at 2:

If both Red and Blue keep playing optimally, the best that Red can get is a pair of parallel 2nd and 4th row ladders in the opposite direction:

Here, stone 10 is connected to the line of blue stones along the top, so Red has no way of connecting right. Red can now push a 4th row ladder from 5, and/or a 2nd row ladder from 13. There is not enough space for Red to immediately perform Tom's move. So unless Red has a ladder escape somewhere to the left of this diagram, or unless there's enough space on the 5th row somewhere to the left of this diagram to perform Tom's move, Red fails to connect to the edge.

Note that this argument does not show that 6th row ladders are categorically impossible. It only shows that the "usual" notion of ladder does not work. It is conceivable that 6th row ladders are possible under additional assumptions. For example, there might be a notion of 6th row ladder that requires additional space on the 7th row to its right, or on the 5th row to its left. It is currently unknown whether any viable notion of 6th row ladder exists.

For 7th row ladders the situation is even worse. As explained in open problems about edge templates, no amount of space under the ladder (even if we demand that the entire 5th row is clear) is known to guarantee a red connection if Blue just ignores the ladder and plays elsewhere. Thus, it is possible that 7th row ladders do not even exist in theory. Of course they do not occur in practice either.

Second-to-fourth row switchbacks

Definition of switchback

Informally, a 2nd-to-4th row switchback is a pattern that allows the attacker to turn around a 2nd row ladder into a ladder on the 4th row in the opposite direction. For example, in the following situation, suppose ladder stone marked "1" is connected to the top, with Blue to move.

Red pushes the 2nd row ladder to d3, the breaks at f3:

At this point, Blue is forced to play 8, and then a new ladder starts in the opposite direction:

In this example, the ladder reconnects to Red's original group, although in general this does not need to be the case (even if the switchback doesn't connect, Red has just created a parallel edge 4 cells from the original edge - a large advantage for Red in any case).

To formalize the concept of a 2nd-to-4th row switchback, consider a 2nd row ladder.

L2:

This is the same ladder as defined in the section of second-row ladders above; only this time, Red's goal will be slightly different. To explain Red's goal, we show a slightly larger area around L2:

This time, Red's goal will be to do at least one of the following two things: either connect the red ladder stone to the edge, or else, occupy the cell marked "a" with a red stone that is connected to the edge, without using the cells marked "b" or any cells to their left. We refer to this as the switchback condition. We also call "a" the switchback cell and "b" the gap cells. With this in mind, we now give the definition of a 2nd-to-4th row switchback template.

Definition. A 2nd-to-4th row switchback template (or simply 2-to-4 switchback) is given by the following data. It is a pattern P with a left boundary of shape

and satisfying the following axiom: L2+↑+↑+n+P satisfies the switchback condition for all n ≥ 0.

As usual, a candidate is a pattern that has the correct shape, but is not (yet) known to be a valid switchback template.

As in previous sections, we write L2+↑+↑+n+P for the pattern obtained from P by moving the four hexes marked "+" to the left by n columns (leaving 4 rows of empty space), then removing the top two cells marked "+" (they are not part of the pattern) and replacing the remaining cells marked "+" by L2. Note that the switchback cell "a" and gap cells "b" are not part of L2. They are simply three cells on the board whose position is defined relative to L2. Depending on the value of n, they may or may not end up being inside the pattern P.

Characterization of switchbacks

The following theorem gives a finite necessary and sufficient condition for a pattern to be a switchback.

Theorem 12. Given a candidate P for a 2-to-4 switchback. Then P is a valid 2-to-4 switchback if and only if L2+↑+↑+P satisfies the switchback condition.

Proof. The left-to-right implication is trivial. For the opposite implication, we will show, by induction on n, that L2+↑+↑+n+P satisfies the switchback condition for all n ≥ 0. The base case n = 0 holds by assumption. Now suppose that the claim is true for some n. To show the claim for n+1, consider the position L2+↑+↑+n+1+P. It looks as follows, with n more empty columns and P appended on the right:

Here, the ladder stone is marked "1". Blue has no choice but to push the ladder, and Red also pushes:

At this point, the induction hypothesis guarantees that Red can either connect 3 to the edge, or else that Red can occupy and connect the switchback cell "a" while keeping "b" empty:

If 3 is connected to the edge, then so is 1, and we are done. Otherwise, "a" is connected to the edge and "b" is empty. Thus, the board looks like this, with "a" now acting as a ladder stone:

Since Red's stones on the 2nd row are already connected to the top, and 1 is connected to the bottom, Blue has no choice but to respond at 2. Then Red can play 3.

Now the switchback condition for L2+↑+↑+n+1+P is satisfied, proving the theorem. □

Examples

Any 2nd row ladder escape template trivially also works as a switchback template (with the location of the cells marked "+" adjusted as necessary; they may need to be moved to the left if there isn't space for the two additional "+"s in the pattern). Since such a template escapes 2nd row ladders outright, there is no need for the second part of the switchback condition.

The following are examples of 2nd-to-4th row switchback templates that are not second row ladder escapes. They are minimal.

In the last two templates, the shaded hex is not part of the template, and can be occupied by Blue. The following template is useful for obtuse corners:

In the following template, the stone marked "↓" is assumed to be connected to the bottom edge, but the connection is not shown. The blue stone is not technically part of the pattern; however, if this cell were empty, the pattern would already work as a 2nd row ladder escape.

Third-to-fifth row switchbacks

Definition of switchback

The definition of 3rd-to-5th row switchbacks is similar to that of 2nd-to-4th row switchbacks. Consider a 3rd row ladder.

L3:

We define the locations of the switchback cell "a" and gap cells "b" relative to L3:

Again, the switchback condition states that with Blue to move, Red can either connect the ladder stone to the edge, or else Red can occupy the switchback cell and connect it to the edge, without using the gap cells or anything to their left.

Definition. A 3nd-to-5th row switchback template (or simply 3-to-5 switchback) is given by the following data. It is a pattern P, open on the left with boundary

and subject to the requirement that L3+↑+↑+n+P satisfies the switchback condition for all n ≥ 0.

Characterization of switchbacks

The following theorem gives a finite necessary and sufficient condition for a pattern to be a switchback. It is analogous to the corresponding theorem for 3rd row ladders.

Theorem 13. Given a candidate P for a 3-to-5 switchback. Then P is a valid 3-to-5 switchback if and only if L3+↑+↑+P and L3+↑+↑+1+P satisfy the switchback condition.

Proof. The left-to-right implication is trivial. For the opposite implication, we will show, by induction on n, that L3+↑+↑+n+P satisfies the switchback condition for all n ≥ 0. The base cases n = 0 and n = 1 hold by assumption. Now suppose that the claim is true for n and n+1. To show the claim for n+2, consider the position L3+↑+↑+n+2+P. It looks as follows, with n more empty columns and P appended on the right:

The ladder stone is marked "1". As usual for 3rd row ladders, Blue must either push or yield, or else Red will connect to the edge outright. If Blue pushes, then so does Red:

By induction hypothesis, L3+↑+↑+n+1+P satisfies the switchback condition, so either Red can connect to the edge, or else Red can get a new ladder stone 1 in the switchback cell for L3+↑+↑+n+1+P, which allows Red to satisfy the switchback condition for L3+↑+↑+n+2+P with stone 3.

The other option is for Blue to yield. (We will see later that when n is large enough, yielding in this situation is actually a terrible idea for Blue, since it will allow Red to use P to connect to the edge. But this is not relevant for the current proof).

Now by the induction hypothesis, L3+↑+↑+n+P satisfies the switchback condition, so either Red can connect to the edge, or else Red can get a new ladder stone 1 in the switchback cell for L3+↑+↑+n+P. This allows Red to satisfy the switchback condition for L3+↑+↑+n+2+P with stone 5.

This finishes the proof of the theorem. □

One may ask whether every 3-to-5 switchback template also works as a 2-to-4 switchback template. This is indeed the case at sufficient distance, due to the following jumping lemma.

Lemma 14 (2nd to 3rd row switchback jump). Any 3-to-5 switchback template is also a 2-to-4 switchback template at distance 4 or greater. More specifically, if P is a 3-to-5 switchback template, then ↑+4+P is a 2-to-4 switchback template.

Proof. Suppose P is a 3-to-5 switchback template, and consider Q = ↑+4+P. By Theorem 12, we must show that L2+↑+↑+Q satisfies the switchback condition. The position L2+↑+↑+Q looks like this, with P attached on the right:

After Blue pushes the ladder at 2, Red plays 3, which is essentially Tom's move. While this move is not sufficient to connect Red to the edge, it creates enough trouble to allow Red to get the desired switchback in the presence of P.

Let us consider Blue's options. If Blue moves outside the area marked "x", Red simply pushes the ladder and connects, using 3 as a ladder escape.

If Blue moves in any of the cells marked 4, Red gets the switchback without using P:

If Blue moves at 4, Red also gets the switchback without using P:

If Blue moves at 4, Red also gets the switchback without using P. Note that 3 is connected by edge template III2b.

If Blue moves at 4, Red also gets the switchback without using P. Note that 3 is connected to the edge by double threat.

This leaves only one option for Blue. If Blue moves at 4, then Red responds as follows:

By hypothesis, since P is a 3-to-5 switchback template, Red can either connect 3 to the edge, or else get a connected red stone at "a", with "b" empty:

In either case, 7 is connected to the edge, so Red has the desired switchback. □

Corollary 15. In a 3rd row ladder at distance 5 or greater to a 3-to-5 switchback, Blue cannot yield.

Proof. If Blue yields, then Red can switch back the resulting 2nd row ladder to the 4th row by the previous lemma. This will reconnect to Red's original 3rd row ladder, and therefore connect Red to the edge. In a diagram:

Note that Blue must play 6 for the same reason as in the lemma. Since Red will either connect 5 or "a" to the edge, 7 is also connected. Rather than just giving Red a switchback, 7 is actually connected to 1 by a crescent. □

Here is another interesting fact about 3-to-5 switchbacks. Given enough space, the defender of a 3rd row ladder cannot yield without giving the attacker a switchback.

Theorem 16. Given enough space to the right of a 3rd row ladder and two empty rows above it, if the defender tries to yield, the attacker can achieve a 3-to-5 switchback without requiring any addtional stones.

Proof. Let 1 be the ladder stone of a 3rd row ladder, and assume there is at least as much space as indicated in the following diagram.

If Blue yields at 2, then Red can play as follows:

If Blue's next move is outside of the area marked "x", then Red connects to the edge outright, as follows:

Therefore, Blue must move in one of the hexes marked "x" above. This leaves nine possible moves for Blue.

If Blue moves at one of the hexes marked 6 below, then Red connects by edge template IV2b:

If Blue moves at 6, then Red pushes the second row ladder twice and connects by Tom's move:

If Blue moves at 6, then Red gets 2nd and 4th row parallel ladders, which connect by Tom's move:

If Blue moves at 6, then Red connects by a crescent and Tom's move:

If Blue moves at 6, then Red connects by crescent and ziggurat:

If Blue moves at 6, then Red connects by shopping cart and Tom's move:

If Blue moves at 6, the situation is almost identical:

Note that in all cases so far, Red connected outright, i.e., didn't need a switchback. The final remaining possibility is for Blue to move at 6 in the following diagram. Then Red gets the switchback. Note that 7 is connected to the edge by edge template IV2b.

This finishes the proof of the theorem. □

Examples

Every 3rd row ladder escape template is also a 3-to-5 switchback template (possibly with the location of the column of "+"s adjusted), but it need not be minimal. Here are some examples of 3-to-5 switchback templates that are not 3rd row ladder escapes.

The shaded cell is not part of the template and can be occupied by Blue.

Second and fourth row parallel ladders

Definition of ladder

Parallel ladders, especially on the 2nd and 4th rows, are quite common in Hex. For example, consider this situation, with Blue to move and the Red stone connected to the top:

Play may proceed as follows:

Now Red has a choice: she can either continue pushing the 4th row ladder from 2, or the 2nd row ladder from 6. However, having parallel ladders puts Red in a stronger position than having a 2nd row ladder or a 4th row ladder alone. As we will see, there exist ladder escape templates than can escape a parallel ladder, but can neither escape a 2nd row ladder nor a 4th row ladder on its own.

Note. Unlike with single-row ladders, in the case of a parallel ladder, Red actually has a choice whether to push the 2nd row ladder or the 4th row ladder. For this reason, our formal definition of a parallel ladder follows a slightly different approach than that we took for single-row ladders above. Whereas above, we always assumed that Blue was next to move (and the ladder stone was already in a pushing position), here, we will assume that Red is next to move. This affects the definition of the ladder pattern, in that the ladder stones do not yet have empty space below them.

Definition. A parallel ladder on the 2nd and 4th rows, or 2-4 parallel ladder for short, is a pattern like this:

L24:

The red stones on the second and fourth rows are called the ladder stones, and Red's goal is to connect at least one of these stones to the bottom edge, with Red to move first. (We can assume that both ladder stones are already connected to the top). We denote this pattern by L24. There is also a variant of L24 that looks like this:

L24a:

L24 and L24a are equivalent, and for simplicity we will only use L24.

Definition of ladder escape

Definition. An escape template for parallel ladders on the 2nd and 4th rows, or 2-4 parallel ladder escape for short, is a pattern P with a left boundary of shape

satisfying the following axiom: adding a 2-4 parallel ladder at distance n, for any n ≥ 0, allows Red to connect, in the sense that it guarantees the connection of at least one of the red ladder stones to the bottom edge, with Red to move.

As before, a candidate is a pattern that has the correct shape, but is not (yet) known to be a valid escape.

Characterization of ladder escapes

Fortunately, 2-4 parallel ladders are easy to analyze; they are almost as simple as 2nd row ladders. The reason is that, just as for 2nd row ladders, the defender has no choice; he must always push, because as we will see, yielding is not an option. We get a simple and clean theorem with a necessary and sufficient condition for 2-4 parallel ladder escapes.

Theorem 17. Consider a candidate P for a 2-4 parallel ladder escape. Then P is a valid 2-4 parallel ladder escape if and only if L24+P, L24+1+P, and L24+2+P allow Red to connect (with Red to move).

Proof. If the ladder escape is valid, then by definition, L24+n+P allows Red to connect for all n, including n = 0, 1, 2. So the left-to-right implication is trivial. To prove the right-to-left implication, assume L24+P, L24+1+P, and L24+2+P allow Red to connect. We prove by induction that L24+n+P allows Red to connect for all n ≥ 0. The cases n = 0, 1, 2 are true by assumption. Now suppose the claim is true for some n ≥ 2. We must show the claim for n+1. To do so, consider the position L24+n+1+P. The first six columns of this position look like this:

This is followed by n−2 more columns of four empty hexes and by the pattern P. Red starts by pushing the 4th row ladder.

Now consider Blue's possible moves. If Blue moves in any of the hexes marked 2 below (or elsewhere on the board), Red wins outright (i.e., without using the induction hypothesis).

If Blue moves in any of the hexes marked 2 below, Red also wins outright:

If Blue moves at 2 below, Red also wins outright:

This means that the only possible move that is not immediately losing for Blue is to push the 4th row ladder. In this case, Red can respond as follows:

This position allows Red to connect by induction hypothesis, finishing the proof. □

It is clear that every 2nd row ladder escape and every 4th row ladder escape is also an escape for 2nd-and-4th row parallel ladders, since Red can decide to push only the 2nd row ladder, or only the 4th row ladder. In addition, 2nd-to-4th row switchback templates also work as 2-4 parallel ladder escapes. This is intuitively clear, as Red can simply push the 2nd row ladder and switch it back to the 4th row, where it will connect with the 4th row of the parallel ladder. The following theorem proves this more formally, using the definitions.

Theorem 18. Every 2nd-to-4th row switchback template is also a 2-4 parallel ladder escape.

Proof. Assume P is a 2nd-to-4th row switchback template. To show that P is a 2-4 parallel ladder escape, we must show that L24+n+P allows Red to connect with Red to move, for all n ≥ 0. Consider the position L24+n+P, which looks as follows, with an additional n blank columns and P on the right:

Red plays as follows. At this point, since n+P is a 2nd-to-4th row switchback template, Red can either connect 3 to the edge, or get a connected stone at "a" with "b" empty.

This allows Red to connect at least one of the ladder stones, as required. □

Examples

As mentioned above, every 2nd row ladder escape, every 4th row ladder escape, and every 2nd-to-4th row switchback template works as a 2-4 parallel ladder escape. But there are some examples of 2-4 parallel ladder escapes that are none of the above. The most famous of these is Tom's move, which states that a sufficient amount of empty space is enough for a 2-4 parallel ladder to connect to the edge. Specifically, the following is a 2-4 parallel ladder escape template:

Other examples:

Here are some other examples of 2-4 parallel ladder escapes that are neither 2nd nor 4th row ladder escapes nor 2nd-to-4th row switchbacks. They can be shown to be valid by Theorem 17, and are minimal. Unlike Tom's move, these ladder escapes don't require space on the 5th row.

While the following two patterns aren't switchbacks at distance 0 or 1, they do work as 2nd-to-4th row switchbacks at distance 2 or greater.

Third and fifth row parallel ladders

Definition of ladder

Parallel ladders on the 3rd and 5th rows are less common than those on the 2nd and 4th rows, but they can occur. Pushing such ladders is less straightforward, as the defender has more options. Basically, as we will show, if the defender refuses to push, then the attacker can at least get a 2nd row ladder. Moreover, a 2nd-to-4th row switchback template is in that case sufficient for the attacker to connect.

Definition. A parallel ladder on the 3nd and 5th rows, or 3-5 parallel ladder for short, is a pattern like this:

L35:

The red stones on the third and fifth rows are again called the ladder stones, and Red's goal is to connect at least one of these stones to the bottom edge, with Red to move first. We denote this pattern by L35. Just like for 2-4 parallel ladders, there is an equivalent pattern for L35 that looks like this:

L35a:

Definition of ladder escape

Definition. An escape template for parallel ladders on the 3rd and 5th rows, or 3-5 parallel ladder escape for short, is a pattern P with a left boundary of shape

satisfying the following axiom: adding a 3-5 parallel ladder at distance n, for any n ≥ 0, allows Red to connect, in the sense that it guarantees the connection of at least one of the red ladder stones to the bottom edge, with Red to move.

As always, a candidate is a pattern that has the correct shape, but is not (yet) known to be a valid escape.

Characterization of ladder escapes

3-5 parallel ladder escapes are not quite as easy to characterize as those for 2-4 parallel ladders, because the defender has more options. We get the following theorem, which only contains a sufficient condition for a pattern to be a 3-5 parallel ladder escape.

Theorem 19. Consider a candidate P for a 3-5 parallel ladder escape. If L35+P, L35+1+P, ..., L35+3+P allow Red to connect (with Red to move), and if ↑+P is a 2nd-to-4th row switchback template, then P is a valid 3-5 parallel ladder escape.

Proof. We prove by induction that L35+n+P allows Red to connect for all n ≥ 0. The cases n = 0, ..., 3 are true by assumption. Now suppose the claim is true for 0, ..., n, where n ≥ 3. We must show the claim for n+1. To do so, consider the position L35+(n+1)+P. The position looks like this, followed by n−3 additional empty columns and P:

Red starts by pushing the 5th row ladder.

Now consider Blue's possible moves. If Blue moves anywhere except the hexes marked "x", then Red wins outright by bridging from 1 to edge template IV-1a.

If Blue moves in any of the hexes marked 2 below, Red moves at 3 and connects by ziggurat and double threat.

This leaves 10 more moves to consider.

If Blue moves at 2, Red pushes the 3rd row ladder.

Now Blue must either push at "x" or yield at "y" (or else Red will connect immediately). If Blue pushes at "x", then Red has a 3-5 parallel ladder at distance n, which connects by induction hypothesis.

If Blue instead yields at "y", then Red can push the 2nd row ladder and use the switchback to either connect 7 to the edge or get a connected stone at "a". Note that "a" is connected to either 1 or 7 by double threat, so Red connects. (As a matter of fact, Red can do better in this case and get a 2-4 parallel ladder, but it is not needed for the current proof).

If Blue moves at 2, then Red plays as follows and connects by edge template III2e:

If Blue moves at 2, then Red gets a 2nd row ladder that connects via the 2-to-4 switchback at "a". The moves 4 and 5 can also be played in the opposite order without changing the result.

If Blue moves at 2, then Red gets a 2nd row ladder that connects via the 2-to-4 switchback at "a". (In fact, Red can get a 2-4 parallel ladder, but it is not needed in this proof).

If Blue moves at 2, Red connects.

If Blue moves at 2, Red connects

If Blue moves at 2, Red connects by edge template IV2d.

If Blue moves at 2, Red can respond at 3.

Now Blue must respond at "x" or "y", or else Red will connect immediately. If Blue plays at "x", Red gets a 2nd row ladder which connects via switchback at "a". Note that 5 is connected to at least one ladder stone by double threat.

If Blue instead plays at "y", Red also gets a 2nd row ladder which connects via switchback at "a".

If Blue moves at 2, Red connects.

Finally, if Blue moves at 2, Red also connects.

This finishes the proof. □

Examples

Like 2-4 parallel ladders, 3-5 parallel ladders have the property that they can connect to the edge outright if given enough space. There is an analog of Tom's move for 3-5 parallel ladders. The following diagram shows the amount of space required. If Red moves in the cell marked "x", Red can guarantee to connect at least one of the ladder stones marked "1" to the edge. The cell marked "x" is essentially the unique winning move (the only other winning option for Red is to push the 3rd row ladder one more hex before playing "x").

We note that this particular pattern is not technically a 3-5 parallel ladder escape. Without additional empty space on the 6th row, it only escapes 3-5 parallel ladders at distance 0 (as shown) and at distance 1. If the ladder starts further away, Blue has the option of yielding to a 2nd row ladder for which Red would need a 2-to-4 switchback template to connect.

Every 3rd row ladder escape, every 5th row ladder escape, and every 3-to-5 switchback template is also a 3-5 parallel ladder escape. Examples of 3-5 parallel ladder escapes that aren't one of the above are relatively rare, but they do exist. The following are some examples. They have been proved correct using Theorem 19, and they are minimal.

The following template escapes 3-5 parallel ladders, but it is not a 3rd-to-5th row switchback, nor does it escape 3rd, 4th, or 5th row ladders.

The following template escapes 3-5 parallel ladders, but it is not a 3rd-to-5th row switchback, nor does it escape 2nd, 3rd, 4th, or 5th row ladders.

Second and fourth row terraced ladders

Sometimes it can happen that a ladder forms on top of another ladder, with the two rows of attacking stones not yet connected to the edge nor to each other. We call this a terraced ladder. The following is an example of a terraced ladder on the 2nd and 4th rows, with Blue to move:

Although terraced ladders look superficially similar to parallel ladders, they should not be confused. There are two important differences: (1) in a parallel ladder, the two rows of attacking stones are connected to each other, whereas in a terraced ladder, they are not, and (2) in a parallel ladder, the upper ladder is "ahead" of the lower one, whereas in a terraced ladders, the upper ladder is at the same level or behind the lower ladder.

In fact, as we noted above, from the attacker's point of view, having 2nd and 4th row parallel ladders is stronger than having only a 2nd row ladder or only a 4th row ladder. For terraced ladders, the opposite is true: a 2nd and 4th row terraced ladder is weaker than having only a 2nd row ladder or only a 4th row ladder. Nevertheless, despite being relatively weak, terraced ladders can be pushed, and there is a notion of terraced ladder escape at arbitrary distance.

Before we develop the theory of terraced ladders, it is worth noting that terraced ladders from Red's point of view are parallel ladders from Blue's point of view, and vice versa. This can be seen by putting a row of blue stones on top, giving Blue an "edge":

Indeed, from Red's point of view, Red has terraced ladders trying to connect to the bottom edge, whereas from Blue's point of view, Blue has parallel ladders trying to connect to the top edge. The fact that parallel ladders are better for Blue than individual ladders explains why terraced ladders are worse for Red than individual ladders.

Definition of ladder

In a terraced ladder, it is the defender, not the attacker, who decides whether to push the 2nd or 4th row ladder. Since the 4th row ladder can lag behind the 2nd row ladder by an arbitrary distance, there isn't just a single ladder template, but a family of them.

Definition. A 2nd and 4th row terraced ladder is any one of the following patterns:

T(0):

T(1):

T(2):

T(3):

and so on. In general, for k ≥ 1, the pattern T(k) looks like

followed by k−1 columns of

followed by

In these patterns, we refer to Red's stone on the 4th row as the top ladder stone, and to Red's rightmost stone on the 2nd row as the bottom ladder stone. Red's goal is to connect the top ladder stone to the bottom edge, assuming it is Blue's turn first. We can assume that the top ladder stone is already connected to the top edge, but we do not assume that the top and bottom ladder stones are connected to each other.

Definition of ladder escape

Definition. A ladder escape template for 2nd and 4th row terraced ladders, or 2-4 terraced ladder escape for short, is a pattern P with left boundary shaped like this,

This pattern P must satisfy the following axiom: for all k ≥ 0 and all n ≥ 0, T(k)+n+P guarantees a connection of the top ladder stone to the bottom edge, with Blue to move.

As always, a candidate is a pattern that has the correct shape, but is not (yet) known to be a valid escape. If P is such a candidate, schematically of the form

then we write ∗+P to denote the pattern obtained from P by replacing the top two cells marked "+" by empty cells, and adding two new cells marked "+" just to their left. The resulting template is then of the shape required for 4th row ladder escapes.

Characterization of ladder escapes

We will reduce the problem of establishing a terraced ladder escape to finitely many cases. This is done by two lemmas. Lemma 20 states that we only need to consider finitely many values of n (the distance from the bottom ladder stone to the escape). Lemma 21 states that we only need to consider finitely many values of k (the distance from the top ladder stone to the bottom ladder stone). .

Lemma 20. Suppose P is a candidate for a 2-4 terraced ladder escape. Assume T(k)+P, T(k)+1+P, and T(k)+2+P are virtual connections for all k ≥ 0 (i.e., they guarantee a connection of the top ladder stone to the edge, with Blue to move). Moreover, assume that ∗+P escapes 2nd, 3rd, and 4th row ladders (or more precisely, ∗+P escapes 4th row ladders, ↑+∗+P (which has 3 cells marked "+") escapes 3rd row ladders, and ↑+↑+∗+P escapes 2nd row ladders). Then P is a valid 2-4 terraced ladder escape.

Proof. We need to show that T(k)+n+P is a virtual connection for the top ladder stone, for all k,n ≥ 0. We prove this by nested induction, with the outer induction being on n, and the inner induction on k. The base cases n = 0, 1, 2 are true by assumption. Now consider some n ≥ 3, and suppose the claim is true up to n−1. We need to show the claim for n. Consider the position T(k)+n+P, which looks like this:

followed by n−3 additional empty columns and P. Here, our diagram illustrates the case k = 4, but the following arguments are valid for all k ≥ 0. The first observation is that if Blue's next move is outside of the area marked "x", Red connects to the edge immediately by a long crescent and edge template III2b:

Note that this works for all k ≥ 0, although for k = 0 and k = 1, the connection is simpler and does not require a long crescent. Therefore, Blue must move in the area marked "x".

We consider each of Blue's options in turn. If Blue moves just below the top ladder stone, then Red responds by pushing the 4th row ladder. In case k > 0, this leads to the position T(k−1)+n+P, and he claim holds by the inner induction hypothesis:

In case k = 0, the situation is worse for Blue: in this case, Red gets a bona fide 4th row ladder, which ∗+P escapes by assumption:

If Blue moves anywhere else on the 3rd or 4th row, then Red connects the two ladders and gets a second row ladder, which ↑+↑+∗+P escapes by assumption. This works for all k ≥ 0, although for illustration, we show only the case k = 4:

This leaves only 5 possible Blue moves to consider.

If Blue moves at 1, Red gets a 3rd row ladder, which ↑+∗+P escapes. Note that 2 is connected to the top ladder stone by a long crescent (for k ≥ 2) or directly (for k = 0, 1).

If Blue moves at 1, Red gets a 3rd row ladder, which ↑+∗+P escapes by assumption:

Note that there are several alternatives to Blue's move 3, but they all result in a 3rd row ladder for Red.

If Blue moves at 1, Red simply pushes the 2nd row ladder, and we are now in position T(k+1)+n−1+P, which is a virtual connection by the outer induction hypothesis.

If Blue moves at 1, Red gets a 2nd row ladder, which ↑+↑+∗+P escapes by assumption. Note again that 2 is connected to the top ladder stone.

If Blue moves at 1, Red gets a 2nd row ladder.

This finishes the proof of the lemma. □

Having reduced the distance n to finitely many cases, we would now like to reduce the parameter k to finitely many cases as well. The following lemma does this.

Lemma 21. Suppose P is a candidate for a 2-4 terraced ladder escape. Assume T(0)+P, T(1)+P, and T(2)+P are virtual connections (i.e., they guarantee a connection of the top ladder stone to the edge, with Blue to move). Then T(k)+P is a virtual connection for all k ≥ 0.

Proof. The first step in the proof is to show that the following two interior patterns are equivalent. By "interior pattern", we mean that the bottom row of red stones does not have to be a board edge.

B(1):

B(2):

If Red plays first in the region, a red move at x captures the entire region, so x is the only move that Red needs to consider, and its outcome is the same in B(1) and B(2).

If Blue moves first in the region, all of the interior moves (i.e., in unmarked cells) are star-decomposition dominated by x. Therefore, Blue only needs to consider the moves x, y, and z. One can show that each of these three moves (x, y, and z) in region B(2) is equivalent to the corresponding move in region B(1). For example, after Blue moves at x, z dominates all of the interior moves and whoever plays there captures the interior, regardless of whether the region is B(1) or B(2).

A consequence of the fact that regions B(1) and B(2) are equivalent is that all "longer" versions of these regions are also equivalent to B(1), B(2), and each other, i.e.,

B(3):

B(4):

and so on. This is easily proved by induction, because each longer region is obtained from the previous one by replacing a subregion of the form B(1) by B(2), which we already showed to be equivalent.

Next, consider this pattern:

B(0):

We claim that B(1) is at least as good as B(0) for Red, in the sense that anything Red can achieve with B(0), Red can also achieve with B(1). (In fact, B(1) is strictly better for Red than B(0), but that fact is not required for this proof). If Red moves first in the region B(0), the move at x again captures the whole region, and therefore achieves everything Red might hope to achieve in the region. In this case, B(0) and B(1) are equivalent. If Blue moves first, the situation is slightly more complicated. We must show that B(0) is at least as good for Blue as B(1). If Blue plays at x in B(1), then Blue has the corresponding option to move at x in B(0), which works for the same reason as in the proof of the equivalence of B(1) and B(2) above. If Blue plays at z in B(1), Red can respond by pushing the ladder, which creates a position that is literally B(0). If Blue plays at y in B(1), Red can respond at x, and a case distinction shows that no matter how the remaining 3 cells are filled, filling them in the same way in B(0) gives an equivalent position.

Finally, let C(0), C(1), C(2), ... be the same patterns as B(0), B(1), B(2), ..., except with the blue stones removed from the carrier. I.e.:

C(0):

C(1):

C(2):

etc. Note that for all k ≥ 0, C(k) is at least as good for Red as B(k). Because if the neighboring cells we removed from the templates are in fact occupied by Blue, then C(k) is the same as B(k); otherwise, if they are empty or Red, it can only help Red.

In particular, since each C(k) is at least as good for Red as B(k), and each B(k) is at least as good as B(0) = C(0), it follows that if Red wins any position containing C(0), then Red also wins the corresponding position containing C(k).

The final step in the proof is now easy. Simply observe that each T(k+2) is obtained from T(2) by replacing a subpattern of the form C(0) by C(k). Therefore, in any context P where T(2)+P is winning for Red, T(k+2)+P is also winning for Red. Combining this with the remaining two base cases T(0)+P and T(1)+P, we get the lemma. □

By combining the previous two lemmas, we obtain a sufficient condition for the validity of terraced ladder escapes.

Theorem 22. Consider a candidate P for a 2-4 terraced ladder escape. Assume T(k)+n+P is a virtual connection for n=1,2,3 and k=1,2,3 (nine possibilities). Moreover, assume that ∗+P escapes 2nd, 3rd, and 4th row ladders (or more precisely, ∗+P escapes 4th row ladders, ↑+∗+P (which has 3 cells marked "+") escapes 3rd row ladders, and ↑+↑+∗+P escapes 2nd row ladders). Then P is a valid 2-4 terraced ladder escape.

Proof. By Lemma 21, T(k)+n+P is a virtual connection for n=1,2,3 and all k ≥ 0. Therefore, the hypothesis of Lemma 20 is satisfied, and thus P is valid. □

Non-examples

Since terraced ladders are weaker than 4th row ladders, any terraced ladder escape is also a 4th row ladder escape. The question then becomes: which 4th row ladder escapes are not terraced ladder escapes? Most, but not all, of the examples of 4th row ladder escapes given above also escape terraced ladders.

The following patterns escape 4th row ladders but do not escape terraced ladders:

@@ Line 1: / Line 1: @@
-The object of this page is to formalise precisely what it means for a pattern to be a ladder escape. To do this we first need to formalise what it means to be a ladder! After we have done this, we can formalise what it means to be a ladder escape, but now there is a catch. A ladder escape (say, to make things precise, a 4th row ladder escape) is supposed to give the attacker a guarantee that their 4th row ladder will be able to connect to the edge, however far away it is. So strictly speaking to check that a pattern is a 4th row ladder escape we need to check that red can connect to the edge from an *infinite set* of positions (the ladder can be as far away as you like). This raises the issue of how one can possibly check in a finite time whether a given pattern is a 4th row ladder escape.
+The object of this page is to formalise precisely what it means for a pattern to be a ladder escape. To do this, we first formalise what it means to be a ladder.
-This issue is resolved on this page, for 2nd, 3rd and 4th row ladders. It might be possible to resolve it for 5th and 6th row ladders but these have no practical use and it will take a lot of work, so I have not done this. For 7th row ladders we run into a new difficulty -- blue can simply ignore the ladder and play near the escape, because no appropriate 6th row edge template seems to be known which will connect an ignored 7th row ladder to the edge. This presents a theoretical obstruction which I do not know how to resolve; I think that in theory there could be no 7th row ladder escapes at all!
+Informally, a ladder escape (say, a 4th row ladder escape) is supposed to give the attacker a guarantee that their 4th row ladder will be able to connect to the edge, no matter how far away from the ladder escape the ladder starts. So strictly speaking, to check that a pattern is a 4th row ladder escape, we must check that the attacker can connect to the edge from an ''infinite set'' of positions. This raises the issue of how one can check in a finite time whether a given pattern is a 4th row ladder escape.
-== Ladders ==
+This issue is resolved on this page for 2nd, 3rd, 4th, and 5th row ladders. It might be possible to resolve it for 6th row ladders but this has not yet been done, partly because such ladders are of little practical use. For 7th row ladders we run into a new difficulty – Blue can simply ignore the ladder and play near the escape, because no appropriate 6th row edge template seems to be known which will connect an ignored 7th row ladder to the edge. This presents a theoretical obstruction which is currently unresolved. It may in theory be that there are no 7th row ladders at all.
-There is no issue at all with defining a 2nd row ladder: it looks like this.
+For the purpose of our analysis, we assume that all ladders move from left to right along the red bottom edge, with Red being the attacker. Of course, the analogous analysis also applies to ladders moving in the opposite direction or along different edges.
-<hexboard size="2x8"
+The analysis of 2nd–4th row ladders on this page was originally contributed by the user [[User:Wccanard|Wccanard]] in 2016.
+'''A note on terminology.''' The usual definition of a ''template'' is a pattern that has a stated property (for example, being [[virtual connection|connected]]) and is also minimal with that property. In other words, a template is usually defined by two properties: validity and minimality. For the purpose of ''this'' page, we are mostly concerned with validity. Since it would be awkward to write "template but not necessarily minimal" throughout all of the definitions and proofs on this page, we adopt the convention, on this page only, that "template" means a pattern that is valid but not necessarily minimal. We will then speak of a "minimal template" when necessary.
+== Algebraic notation ==
+Before we start, let us introduce some notation that will be useful.
+=== Open patterns ===
+A ''pattern'' is a set of cells, each of which may be empty or occupied by a stone of either color. In this article, we will only be concerned with patterns that include a red board edge. A pattern is ''open on the left'' if it comes with some cells marked "+" on its left side. No cells to the left of those "+"s may be part of the pattern. A pattern is ''open on the right'' if it comes with some cells marked "−" on its right side. No cells to the right of those "−"s may be part of the pattern. A pattern is ''open on both sides'' if it is open on the left and on the right. A pattern is ''closed'' if it is not open on either side. For example, the following four patterns are open on the right, open on both sides, open on the left, and closed, respectively:
+<hexboard size="3x3"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="R b1 E -:(c1 c2 c3)"
+  /> <hexboard size="3x3"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="all"
+  contents="R b1 B c1 E +:(a1 a2 a3) -:(c2 c3)"
+  /> <hexboard size="3x2"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 b1"
+  contents="R b3 E +:(a2 a3)"
+  /> <hexboard size="3x4"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="R b1 c1 d3 B d1"
+  />
+The cells labelled "+" (if any) are called the ''left boundary'' of the pattern, the cells labelled "−" are called its ''right boundary'', and the ''carrier'' of a pattern consists of all cells that are part of the pattern (empty or not), except the boundaries.
+=== Addition ===
+Suppose P is a pattern that is open on the right, Q is a pattern that is open on the left, and the right boundary of P has the same number of cells and shape as the left boundary of Q. Then we write P+Q for the pattern obtained by joining P and Q along their common boundary. More specifically, P+Q is obtained as follows: delete the right boundary from P and the left boundary from Q. Then glue the patterns P and Q together along the line where the boundaries were deleted from each. For example:
+<hexboard size="3x3"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="R b1 E -:(c1 c2 c3)"
+  /> + <hexboard size="4x4"
+   float="inline"
     coords="hide"
-    contents="R a1 R b1 R c1 R d1 R 2:e1 R 4:f1 B a2 B b2 B c2 B 1:d2 B 3:e2"
+   edges="bottom"
-   />
+   visible="-a1 d3 d4"
+    contents="E +:(a2,a3,a4) R c1 d1"
+   /> = <hexboard size="4x5"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2 a3 b1 e3 e4"
+  contents="R b2 d1 e1"
+  />
+It is important to note that the carrier of P+Q consists of just the carriers of P and Q, ''without'' the boundary cells that have been deleted. The purpose of the boundary cells "+" and "−" is just to indicate where the patterns will be attached. It is possible to add more than two patterns, for example:
+<hexboard size="3x3"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="R b1 E -:(c1 c2 c3)"
+  /> + <hexboard size="3x3"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="all"
+  contents="R b1 B c1 E +:(a1 a2 a3) -:(c2 c3)"
+  /> + <hexboard size="3x2"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 b1"
+  contents="R b3 E +:(a2 a3)"
+  /> = <hexboard size="3x4"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="R b1 c1 d3 B d1"
+  />
-There is a minor issue with defining 3rd and higher row ladders though. We want a definition which is useful in practice and not too restrictive. For example we surely want this to be a third row ladder:
+Note that addition is only well-defined if P and Q can be glued together without their carriers overlapping. We will be careful to ensure that this is always the case. However, the addition is associative, i.e., (P + Q) + R is well-defined if and only if P + (Q + R) is well-defined, and in that case, they are equal.
-<hexboard size="4x10"
+=== The shift operation ===
+If P is a pattern that is open on the left, we write 1 + P for the pattern obtained from P by shifting its left boundary one column to the left, and adding a column of empty cells where the boundary used to be. More generally, for any integer ''n'' ≥ 0, we write ''n'' + P for iterating this operation ''n'' times, i.e., for shifting the left boundary of P to the left by ''n'' columns and filling the additional space with empty cells. For example:
++ <hexboard size="4x4"
+   float="inline"
     coords="hide"
-    contents="R c1 B b2 R c2 R d2 R e2 R f2 R 2:g2 R 4:h2 B a3 B b3 B c3 B d3 B e3 B 1:f3 B 3:g3 B a4 B d4"
+   edges="bottom"
-    />
+   visible="-a1 d3 d4"
-even though there are a couple of blue pieces on the first row. It is intuitively clear (and also provably true) that these blue pieces cannot be of any help to blue (they can never play a crucial role in any blue connection). So although we want a 3rd row ladder (moving from left to right, say) to have no stones on the first three rows to the right of the ladder (until we reach the escape of course), we do not want to also guarantee that there are no stones on the first row to the left of the ladder.
+    contents="E +:(a2,a3,a4) R c1 d1"
+    /> = <hexboard size="4x5"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 b1 e3 e4"
+  contents="E +:(a2 a3 a4) R d1 e1"
+  />
-Here is the definition I came up with. A '''third row ladder''' is a pattern like this:
-<hexboard size="3x2"
++ <hexboard size="4x4"
+   float="inline"
     coords="hide"
-    contents="E *:a1 R b1 B a2"
+   edges="bottom"
-   />
+   visible="-a1 d3 d4"
-The asterisk is not part of the pattern. The red piece can be thought of as being connected to the top. The key part of the definition is that we are guaranteeing the triangle of three empty hexes under the red piece. This is a minimal requirement, because for example if one of these pieces were filled:
+    contents="E +:(a2,a3,a4) R c1 d1"
-<hexboard size="3x2"
+   /> = <hexboard size="4x8"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 b1 c1 d1 e1 h3 h4"
+  contents="E +:(a2 a3 a4) R g1 h1"
+  />
+Note that empty cells are only added to the height of the boundary. For a pattern that is open on the right, we can do exactly the same thing on the other side, i.e., P + ''n'' is obtained by shifting the right boundary to the right by ''n'' columns, filling the additional space with empty cells. For example:
+<hexboard size="3x3"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2 b1"
+  contents="R c1 E -:(c2 c3)"
+  /> + 1 &nbsp;&nbsp;&nbsp; = <hexboard size="3x4"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2 b1 d1"
+  contents="R c1 E -:(d2 d3)"
+  />
+<hexboard size="3x3"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2 b1"
+  contents="R c1 E -:(c2 c3)"
+  /> + 4 &nbsp;&nbsp;&nbsp; = <hexboard size="3x7"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1 a2 b1 d1 e1 f1 g1"
+  contents="R c1 E -:(g2 g3)"
+  />
+We call this the ''shift operation''. Note that it is associative: If P and Q have matching boundaries, then (P + ''n'') + Q = P + (''n'' + Q).
+=== The reduce operation ===
+If P is a pattern that is open on the left, we write ↑ + P for the pattern obtained from P by erasing the topmost "+" cell from its left boundary. The cell that formerly contained the "+" is no longer part of the pattern (i.e., it is not replaced by an empty cell). For example:
+↑ + <hexboard size="4x4"
+   float="inline"
     coords="hide"
-    contents="E *:a1 R b1 B a2 B a3"
+   edges="bottom"
-    />
+   visible="-a1 d3 d4"
-then in reality the game could look like this:
+    contents="E +:(a2,a3,a4) R c1 d1"
-<hexboard size="4x6"
+    /> = <hexboard size="4x4"
+   float="inline"
     coords="hide"
-    contents="R a1 B b1 B c1 B d1 B e1 B f1 R a2 R b2 B a3 B a4"
+    edges="bottom"
+   visible="-a1 a2 d3 d4"
+   contents="E +:(a3,a4) R c1 d1"
     />
-and blue can block the ladder with this move.
-<hexboard size="4x6"
+We call this the ''reduce operation''. The shift and reduce operations can be combined with each other and with addition of patterns. For instance, ''n'' + ↑ + ''m'' + P is the pattern obtained from P by first shifting its left boundary by ''m'' cells to the left, then reducing the size of that boundary by one cell, and then shifting it by another ''n'' cells to the left. For example:
++ ↑ + 3 + <hexboard size="4x4"
+   float="inline"
     coords="hide"
-    contents="R a1 B b1 B c1 B d1 B e1 B f1 R a2 R b2 B a3 B 1:c3 B a4"
+   edges="bottom"
-   />
+   visible="-a1 d3 d4"
-So that is my working definition of a third row ladder, and it seems to work in practice.
+    contents="E +:(a2,a3,a4) R c1 d1"
+   /> = <hexboard size="4x9"
+  float="inline"
+  coords="none"
+  edges="bottom"
+  visible="-a1--f1 a2--c2 i3 i4"
+  contents="E +:(a3 a4) R h1 i1"
+  />
-The problems compound themselves as we move up the board. Here is my working definition of a 4th row ladder:
+== Second row ladders ==
-<hexboard size="4x3"
+=== Definition of ladder ===
+There is no issue at all with defining a 2nd row ladder. Informally, a 2nd row ladder looks like this:
+<hexboard size="2x8"
     coords="hide"
-    contents="E *:a1 E *:b1 R c1 E *:a2 E *:b2 E *:a3"
+    edges="bottom"
-   />
+    contents="R a1 R b1 R c1 R d1 R 2:e1 R 4:f1 B a2 B b2 B c2 B 1:d2 B 3:e2"
-You see that I have replaced the blue piece with an asterisk (indicating "not part of the pattern"). This is OK because hexes which are not part of the pattern may as well be blue pieces. What I am suggesting here is that for a 4th row ladder to become established we should demand that the triangle with 6 hexes below the laddering piece are all vacant. Note that even filling in one of these can break the ladder: even if we fill in the bottom left corner of the triangle:
-<hexboard size="5x5"
-   coords="hide"
-    contents="B a1 B b1 R c1 B d1 B e1 B a2 B b2 R c2 B a3 B b3 B a4 B a5"
     />
-then blue has this move
+Note that at each point in the ladder, Blue's move is forced. Red can choose to continue pushing the ladder as long as she wants to. We formally define a second row ladder as follows:
-<hexboard size="5x5"
-   coords="hide"
-   contents="B a1 B b1 R c1 B d1 B e1 B a2 B b2 R c2 B a3 B b3 B 1:d3 B a4 B a5"
-   />
-which is easily seen to stop the ladder. To to make the ladder established red needs at a minimum those 6 vacant hexes under their red stone.
-I have said above that I will be restricting my attention to 2nd, 3rd and 4th row ladders, and one of the reasons for this is that I do not know a satisfactory definition of a 5th row ladder. What I do know is that this
+'''Definition.''' A ''second row ladder'' is a pattern like this:
-<hexboard size="5x5"
+L2: <hexboard size="2x2"
     coords="hide"
-    contents="E *:a1 E *:b1 E *:c1 E *:d1 R e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:c3 E *:a4 E *:b4 E *:a5"
+   edges="bottom"
+    contents="R a1 E -:(b1 b2)"
     />
-will '''not''' do, because it turns out that if it is blue to play in the diagram below:
+Here, the red stone is on the second row, and we call it the ''ladder stone''. Red's goal is to connect the ladder stone to the bottom edge. The cell immediately below and to the right of the ladder stone is empty. We denote this pattern by L2.
-<hexboard size="6x15"
-   coords="hide"
-   contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 B n1 B o1 R a2 R b2 R c2 R d2 R e2 R o2 B a3 B b3 B c3 B d3 R o3 B a4 B b4 B c4 R o4 B a5 B b5 R o5 B a6 R o6"
-   />
-then blue can block the ladder with this move:
-<hexboard size="6x15"
-   coords="hide"
-   contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 B n1 B o1 R a2 R b2 R c2 R d2 R e2 R o2 B a3 B b3 B c3 B d3 R o3 B a4 B b4 B c4 R o4 B a5 B b5 B 1:e5 R o5 B a6 R o6"
-   />
-The main line is complex; see http://littlegolem.net/jsp/forum/topic2.jsp?forum=50&topic=669 for example. The conclusion from this is that red needs more space in order to ensure that the ladder is established, and because I am only an amateur hex player I do not really know the minimal amount required or even if one can expect a "natural" answer to this -- perhaps there is more than one kind of 5th row ladder.
-A similar issue arises with 6th row ladders -- I do not know how much space to guarantee red under the ladder stone. And for 7th row ladders I have already explained in [[open problems about edge templates]] that the situation is even worse -- no amount of space under the ladder (even if we demand that the entire 5th row is clear) seems to guarantee a red connection if blue just ignores the ladder and plays elsewhere, from which I deduce that I am struggling to make sense of the notion of a 7th row ladder from a theoretical point of view whilst simultaneously being aware that there is probably essentially no use for it from a practical point of view either.
+=== Definition of ladder escape ===
-== 2nd row ladder escapes ==
+Before we give the formal definition of a second row ladder escape, let us consider an example. The following pattern is an example of a second row ladder escape.
-I have only formulated the definition of an n'th row ladder for 2<=n<=4 above. Let me start this discussion of ladder escapes with a discussion of 2nd row ladder escapes, because here we can see the main points without any of the technicalities about how much space one is supposed to allow under a ladder -- there is no space under the ladder.
-Let me start with an example. An example of a '''second row ladder escape''' is the following pattern.
 <hexboard size="4x4"
     coords="hide"
+   edges="bottom"
+   visible="-a1 b1 a2 d3 d4"
     contents="E *:a1 E *:b1 R c1 R d1 E *:a2 E +:a3 E *:d3 E +:a4 E *:d4"
     />
-Of course directly underneath the pattern is the bottom (red) edge. The pluses indicate where the ladder connects. The reason this is a second row ladder escape is that however far away the ladder is:
+Of course directly underneath the pattern is the bottom (red) edge. The cells marked "+" indicate where the ladder connects. The reason this is a second row ladder escape is that however far away the ladder is,
+<hexboard size="4x8"
-<hexboard size="4x10"
     coords="hide"
-    contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 R i1 R j1 R a2 B b2 B c2 B d2 B e2 B f2 B g2 R a3 R b3 R 1:c3 B j3 B a4 B b4 B j4"
+   edges="bottom"
+   visible="-a1 b1 c1 d1 e1 f1 a2 b2 c2 d2 e2 h3 h4"
+    contents="E *:a1 E *:b1 E *:c1 E *:d1 E *:e1 E *:f1 R g1 R h1 E *:a2 E *:b2 E *:c2 E *:d2 E *:e2 R 1:a3 E *:h3 E *:h4"
     />
-red can guarantee a connection from the ladder stone (marked 1) to the bottom edge. Note that I have replaced asterisks with blue stones -- any hex not in the pattern or the ladder can be thought of as a blue stone, as red is not allowed to move there.
+Red can guarantee a connection from the ladder stone (marked 1) to the bottom edge.
-Let me clarify what the plussed hexes mean: they indicate the last point where the 2nd row ladder is allowed to start. So for example saying that the pattern above is a second row ladder escape means (amongst other things) that red must win the following position:
+Let us clarify what the hexes marked "+" in the ladder escape pattern mean. They indicate the last point where the 2nd row ladder is allowed to start. So for example, saying that the pattern above is a second row ladder escape means (among other things) that Red must win the following position:
 <hexboard size="4x4"
     coords="hide"
+   edges="bottom"
+   visible="-a1 b1 a2 d3 d4"
     contents="B a1 B b1 R c1 R d1 B a2 R 1:a3 B d3 B d4"
     />
-Here we regard stone 1 as connected to the top, and the claim (easily verified) is that even with blue to play, red can connect to the bottom:
+Here, Red's ladder stone is marked "1", and the claim (easily verified) is that even with Blue to play, Red can connect the ladder stone to the bottom:
 <hexboard size="4x4"
     coords="hide"
+   edges="bottom"
+   visible="-a1 b1 a2 d3 d4"
     contents="B a1 B b1 R c1 R d1 B a2 R 5:b2 R 1:a3 B 4:b3 R 3:c3 B d3 B 2:a4 B d4"
     />
+The reason that the pattern is a second row ladder escape is that this escape sequence works even if the ladder is a long way away:
-The reason that the pattern is a second stone ladder escape is that this escape sequence works even if the ladder is a long way away:
+<hexboard size="4x8"
-<hexboard size="4x10"
     coords="hide"
-    contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 R i1 R j1 R a2 B b2 B c2 B d2 B e2 B f2 B g2 R a3 R b3 R 1:c3 B j3 B a4 B b4 B j4"
+   edges="bottom"
+   visible="-a1 b1 c1 d1 e1 f1 a2 b2 c2 d2 e2 h3 h4"
+    contents="B a1 B b1 B c1 B d1 B e1 B f1 R g1 R h1 B a2 B b2 B c2 B d2 B e2 R 1:a3 B h3 B h4"
     />
-Even here red can force a connection to the edge, even if it is blue's move, because blue must keep defending on the first row and red keeps attacking on the second row:
+Even here, Red can force a connection to the edge, even if it is Blue's move, because Blue must keep defending on the first row and Red keeps attacking on the second row,
-<hexboard size="4x10"
+<hexboard size="4x8"
     coords="hide"
-    contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 R i1 R j1 R a2 B b2 B c2 B d2 B e2 B f2 B g2 R a3 R b3 R 1:c3 R 3:d3 R 5:e3 R 7:f3 R 9:g3 B j3 B a4 B b4 B 2:c4 B 4:d4 B 6:e4 B 6:f4 B j4"
+   edges="bottom"
+   visible="-a1 b1 c1 d1 e1 f1 a2 b2 c2 d2 e2 h3 h4"
+    contents="B a1 B b1 B c1 B d1 B e1 B f1 R g1 R h1 B a2 B b2 B c2 B d2 B e2 R 1:a3 R 3:b3 R 5:c3 R 7:d3 R 9:e3 B h3 B 2:a4 B 4:b4 B 6:c4 B 6:d4 B h4"
     />
-and now we are back at the previous position with the ladder right next to the escape, where we have already seen that red can break through to the edge.
+and now we are back at the previous position with the ladder right next to the escape, where we have already seen that Red can break through to the edge. We can now give a more formal definition of a second row ladder escape.
-Now we can guess the general definition. Before I start I should say that for simplicity I won't allow mirror images and all 2nd row ladders should be thought of as moving right and approaching the escape from the left. A '''second row ladder escape''' is the following data. It is a pattern, plus two hexes with plusses in them, such that one of the plussed hexes is on the first row, the other is on the second row up and directly to the left of the first hex, and such that neither of the plussed hexes nor any hex directly to the left of either of the plussed hexes are in the pattern. This data is subject to the following axiom: any position comprising of a second row ladder
+'''Definition.''' A ''second row ladder escape template'' (or simply ''second row ladder escape'') is given by the following data. It is a pattern P that is open on the left (see [[#Algebraic notation|algebraic notation]] above), with a boundary of this shape:
+<hexboard size="2x1"
+  coords="hide"
+  edges="bottom"
+  contents="E +:(a1 a2)"
+  />
+subject to the following axiom: for all ''n'' ≥ 0, the position L2 + ''n'' + P is a [[strong connection|virtual connection]] from the ladder stone (marked 1) to the edge.
+Concretely, this means that any position consisting of a second row ladder L2,
 <hexboard size="2x1"
     coords="hide"
+   edges="bottom"
     contents="R 1:a1"
     />
-followed directly to the right by as many pairs of vacant hexes as you like on the first and second rows:
+followed directly to the right by an arbitrary number (zero or more) of pairs of vacant hexes on the first and second rows,
 <hexboard size="2x1"
     coords="hide"
+   edges="bottom"
     contents=""
     />
-followed by the second row ladder escape pattern (where the ladder slots into the escape by putting the ladder onto the plussed hexes) is an edge template, in the sense that even if it is blue to move, red can guarantee a connection from the ladder stone marked 1 to the edge.
+followed by the second row ladder escape pattern (where the ladder slots into the escape by putting the ladder or rightmost column of vacant hexes onto the hexes marked "+"), allows Red to connect the ladder stone to the edge.
+Terminology and notation: If we have a left-open pattern whose boundary is of the correct shape, but we are not sure whether it satisfies the axiom of a second row ladder escape, then we refer to it as a ''candidate for a second row ladder escape'' (or simply ''candidate'' if the rest is clear from the context). A candidate is ''valid'' if it is actually a ladder escape.
+We also define what it means for a ladder escape template to be minimal.
+'''Definition.''' A 2nd row ladder escape template is ''minimal'' if the following two things are true. First, removing any hex from the pattern, or removing a red stone from the pattern (and replacing it with an empty hex) gives a new pattern which is not a 2nd row ladder escape template any more. And second, if the two hexes directly to the right of the two cells marked "+" are both vacant hexes in the pattern, then moving the cells marked "+" one hex to the right results in a new pattern which is not a 2nd row ladder escape.
+Below, we will use analogous terminology and notations for ladders and ladder escapes on the 3rd and higher rows.
-Notation: let's say that our 2nd row ladder pattern followed by n (an integer >= 0) pairs of vacant hexes is called a "second row ladder at distance n". What we are demanding of our second row ladder escape template is that it becomes an edge template when you slot in a second row ladder at distance n, for all values of n>=0.
+=== Characterization of ladder escapes ===
-This is an interesting definition because it allows the ladder to be an *arbitrary* distance away from the escape, which is of course what we want in practice; there is no reason that the escape should be right next to the ladder. However this means that to check that something is a 2nd row ladder escape we need to check that *infinitely many* patterns are edge templates. How can we do this? Well of course as every hex player knows,
+The definition of a 2nd row ladder escape allows the ladder to be an ''arbitrary'' distance away from the escape, which is of course what we want in practice; there is no reason that the escape should be right next to the ladder. However, this means that we cannot directly use the definition to check that something is a 2nd row ladder escape, because this would require checking that infinitely many patterns are virtual connections. Can we find some finite criterion for checking 2nd row ladder escapes? Fortunately, as every Hex player knows, the answer is yes. We have the following theorem:
-Lemma: a pattern
+'''Theorem 1.''' Consider a candidate P for a 2nd row ladder escape. Schematically:
 <hexboard size="4x4"
     coords="hide"
+   edges="bottom"
     contents="E *:a1 E *:b1 E *:c1 E *:d1 E *:a2 E *:b2 E *:c2 E *:d2 E +:a3 E *:b3 E *:c3 E *:d3 E +:a4 E *:b4 E *:c4 E *:d4"
     />
-(where here the asterisks can indicate any pieces at all)
+(Here the asterisks indicate the [[carrier]] of P, which can contain any stones at all, and can be of any shape or size, as long as it includes no cells to the left of the cells marked "+"). Then P is a valid 2nd row ladder escape if and only if L2+P is a virtual connection for Red.
-is a second row ladder escape if, and only if, replacing the plussed hexes with a second row ladder (at distance zero)
 <hexboard size="4x4"
     coords="hide"
+   edges="bottom"
     contents="E *:a1 E *:b1 E *:c1 E *:d1 E *:a2 E *:b2 E *:c2 E *:d2 R a3 E *:b3 E *:c3 E *:d3 E *:b4 E *:c4 E *:d4"
     />
-is an edge template for red.
-Proof: If the pattern is a second row ladder escape then *by definition* replacing the plusses with a second row distance zero ladder right next to the escape gives an edge template (indeed a second row ladder escape means that this position and infinitely many other positions are an edge template). So this finishes the implication in one direction.
+'''Proof.''' If the ladder escape is valid, then by definition, L2+''n''+P is a virtual connection for all ''n'' ≥ 0, and in particular for ''n'' = 0. This proves the left-to-right implication.
-To go the other way we actually have to play some hex, but it's pretty trivial. Say the pattern becomes an edge template if we insert a second row ladder at distance zero. We now have to prove that the pattern becomes an edge template if we insert a second row ladder at distance n for all n, and this is an easy induction on n, because blue must play directly below red's ladder piece and now red can continue playing the ladder on the second row, and by induction this is an edge template.
+To go the other way we actually have to play some Hex, but it's pretty trivial. We must show that L2+''n''+P is a virtual connection for all ''n''. This is an easy induction on ''n''. For ''n'' = 0, the claim is true by assumption. If ''n'' > 0, then Blue must play directly below Red's ladder stone (or else Red will connect to the edge immediately), and now Red can play a ladder stone at distance ''n''−1 on the second row, which is a virtual connection by induction hypothesis. □
-QED
+=== Examples ===
-Definition: A 2nd row ladder escape template is '''minimal''' if the following two things are true. Firstly, removing any hex from the pattern gives a new pattern which is not a 2nd row ladder escape template any more. And secondly, if the two hexes directly to the right of the two plussed stones are both vacant hexes in the pattern, then moving the plussed hexes one hex to the right results in a new pattern which again is not a 2nd row ladder escape.
+We can use Theorem 1 to prove that all of the following patterns are minimal second row ladder escapes. Most of these templates are taken from [http://www.drking.org.uk/hexagons/hex/templates.html David King's website], and there are several more there. For several of the templates, the corresponding pattern on David King's site is not minimal by our definition; for these templates, we have moved the cells marked "+" to the right to make the template minimal.
-Corollary [cf http://www.drking.org.uk/hexagons/hex/templates.html]: the following positions are minimal second row ladder escapes:
 <hexboard size="2x2"
     coords="hide"
+   edges="bottom"
     contents="E +:a1 E +:a2 R b2"
     />
 <hexboard size="2x2"
     coords="hide"
+   edges="bottom"
     contents="E +:a1 R b1 E +:a2"
     />
 <hexboard size="3x3"
     coords="hide"
+   edges="bottom"
+   visible="-a1"
     contents="E *:a1 R b1 E +:a2 E +:a3"
     />
-(note that the corresponding pattern on Dr King's site is not minimal; I have moved the plusses)
 <hexboard size="3x3"
     coords="hide"
+   edges="bottom"
+   visible="-a1"
     contents="E *:a1 R c1 E +:a2 E +:a3"
     />
-(note that the corresponding pattern on Dr King's site is not minimal; I have moved the plusses)
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 d3 d4"
+   contents="E *:a1 E *:a2 E +:a3 E +:a4 E *:b1 R d1 R d2 E *:d3 E *:d4"
+   />
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 d3 d4"
+   contents="E +:a3 E +:a4 R c1 R d1"
+   />
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 c1 f1 a2"
+   contents="E *:a1 E *:b1 E *:c1 R d1 E *:f1 E *:a2 E +:a3 E +:a4"
+   />
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 c1 f1 a2"
+   contents="E *:a1 E *:b1 E *:c1 R e1 E *:f1 E *:a2 E +:a3 E +:a4"
+   />
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 d4 d5"
+   contents="E *:a1 *:a2 *:b1 *:d4 *:d5 +:a4 +:a5 R c1 R d2"
+   />
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 a3 b1 b2 c1 c2 d1 e1 i1 j1 j2"
+   contents="E *:a1 E *:a2 E *:a3 E +:a4 E +:a5 E *:b1 E *:b2 E *:c1 E *:c2 E *:d1 E *:e1 R g1 E *:i1 E *:j1 E *:j2"
+   />
+<hexboard size="5x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1"
+   contents="E +:(a4 a5) R c1 d1"
+   />
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 a3 b1 b2 c1 c2 d1 e1 i1 j1 j2"
+   contents="E *:a1 E *:a2 E *:a3 E +:a4 E +:a5 E *:b1 E *:b2 E *:c1 E *:c2 E *:d1 E *:e1 R g1 E *:i1 E *:j1 E *:j2 R h1 S g2"
+   />
+The shaded cell is not part of the template, and can be occupied by Blue.
+In the below templates, the stone marked "↓" indicates a stone connected to the bottom edge, but the connection is not shown. The connection from 10 to the edge must not use any of the empty hexes in the pattern.
 <hexboard size="2x3"
     coords="hide"
-    contents="E +:a1 R 10:c1 E +:a2 E *:b2 E *:c2"
+   edges="bottom"
+   visible="-b2 c2"
+    contents="E +:a1 R ↓:c1 E +:a2 E *:b2 E *:c2"
     />
-(here and below the 10 indicates a stone connected to the bottom edge, but the connection is not shown)
 <hexboard size="3x4"
     coords="hide"
-    contents="E *:a1 E +:a2 R 10:d2 E +:a3 E *:b3 E *:c3 E *:d3"
+   edges="bottom"
+   visible="-a1 b3 c3 d3"
+    contents="E *:a1 E +:a2 R ↓:d2 E +:a3 E *:b3 E *:c3 E *:d3"
     />
 <hexboard size="4x4"
     coords="hide"
-    contents="E *:a1 E *:b1 E *:c1 R 10:d1 E *:a2 E *:d2 E +:a3 E *:c3 E *:d3 E +:a4 E *:b4 E *:c4 E *:d4"
+   edges="bottom"
+   visible="-a1 b1 c1 a2 d2 c3 d3 b4 c4 d4"
+    contents="E *:a1 E *:b1 E *:c1 R ↓:d1 E *:a2 E *:d2 E +:a3 E *:c3 E *:d3 E +:a4 E *:b4 E *:c4 E *:d4"
     />
+== Third row ladders ==
+=== Definition of ladder ===
+There is a minor issue with defining ladders on the 3rd and higher rows. We want a definition that is useful in practice and not too restrictive. For example, we surely want this to be a third row ladder:
+<hexboard size="4x10"
+   coords="hide"
+   edges="bottom"
+   contents="R c1 B b2 R c2 R d2 R e2 R f2 R 2:g2 R 4:h2 B a3 B b3 B c3 B d3 B e3 B 1:f3 B 3:g3 B a4 B d4"
+   />
+even though there are a few blue stones on the first row. It is intuitively clear (and also provably true) that these blue stones cannot be of any help to Blue (they can never play a crucial role in any blue connection). So although we want a 3rd row ladder to have no stones on the first three rows to the right of the ladder (until we reach the escape), we do not want to also guarantee that there are no stones on the first row to the left of the ladder. We formally define third row ladders as follows.
+'''Definition.''' A ''third row ladder'' is a pattern like this:
+L3: <hexboard size="3x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="R b1 E -:(c1--c3)"
+   />
+The red stone is again called the ''ladder stone'', and Red's goal is to connect the ladder stone to the bottom edge. We denote this pattern by L3.
+The key part of the definition is that we are guaranteeing the triangle of three empty hexes under the red ladder stone. This is a minimal requirement, because for example if one of these cells were filled,
+<hexboard size="3x2"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="E *:a1 R b1 B a2 B a3"
+   />
+then in reality the game could look like this,
 <hexboard size="4x6"
     coords="hide"
-    contents="E *:a1 E *:b1 E *:c1 R d1 E *:f1 E *:a2 E +:a3 E +:a4"
+   edges="all"
+    contents="R a1 B b1 B c1 B d1 B e1 B f1 R a2 R b2 B a3 B a4"
     />
+and Blue can block the ladder with this move.
 <hexboard size="4x6"
     coords="hide"
-    contents="E *:a1 E *:b1 E *:c1 R e1 E *:f1 E *:a2 E +:a3 E +:a4"
+   edges="all"
+    contents="R a1 B b1 B c1 B d1 B e1 B f1 R a2 R b2 B a3 B 1:c3 B a4"
     />
-and so on and so on (these templates are all taken from Dr King's site, and there are several more there).
-== Third row ladder escapes ==
+=== Definition of ladder escape ===
-We have seen a lot of the formalism of ladder escapes in the above section on second row escapes. However there is a new twist with third row ladder escapes, because blue can defend against a third row ladder in more than one way: blue can at some stage decide to drop to the second row.
+We have seen a lot of the formalism of ladder escapes in the above section on second row escapes. However there is a new twist with third row ladder escapes, because Blue can defend against a third row ladder in more than one way: Blue can at some stage decide to [[ladder handling|yield]] to the second row. The following definition is unsurprising.
-The following definition is unsurprising: A '''third row ladder escape''' is the following data: it is a pattern, and three plussed hexes (none of these plussed hexes are part of the pattern) going from the first to the third row, up and left as in the below picture, which is sitting on the bottom row of a hex board:
+'''Definition.''' A ''third row ladder escape'' is given by the following data. It is a pattern P that is open on the left, with a boundary of the shape
+<hexboard size="3x1"
+  coords="hide"
+  edges="bottom"
+  contents="E +:(a1--a3)"
+  />
+To be a third row ladder escape, the pattern must satisfy the property that for all ''n'' ≥ 0, L3 + ''n'' + P is a virtual connection from the Red ladder stone to the bottom edge, with Blue to move.
+Like for second row escapes, a pattern that has the required shape for a ladder escape, but it is not (yet) known to be a valid ladder escape, is called a ''candidate''.
+In pictures, for the pattern
 <hexboard size="3x3"
     coords="hide"
+   edges="bottom"
     contents="E +:a1 E *:b1 E *:c1 E +:a2 E *:b2 E *:c2 E +:a3 E *:b3 E *:c3"
     />
-(here the starred hexes can be anything, they are just part of a general pattern, which could certainly go above the third row if necessary). The data must have the property that no hex directly to the left of any of the plussed hexes can be part of the pattern. And the data is subject to the axiom that for any n>=0, if you insert a third row ladder at distance n onto the pattern at the three plussed hexes, the resulting position is an edge template connecting the ladder stone to the bottom edge.
+(where the carrier is schematically indicated by stars) to be a 3rd row ladder escape, it must give rise to a virtual connection when we attach a 3rd row ladder at distance 0,
-In pictures, our pattern above (still marked with stars), if it is a 3rd row ladder escape, must give rise to an edge template when we attach a 3rd row template at distance 0
 <hexboard size="3x4"
     coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
     contents="B a1 R 1:b1 E *:c1 E *:d1 B a2 E *:c2 E *:d2 E *:c3 E *:d3"
     />
-or at distance 1
+or at distance 1,
 <hexboard size="3x5"
     coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
     contents="B a1 R b1 E *:d1 E *:e1 B a2 E *:d2 E *:e2 E *:d3 E *:e3"
     />
-or at distance 6
+or at distance 6,
 <hexboard size="3x10"
     coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
     contents="B a1 R b1 E *:i1 E *:j1 B a2 E *:i2 E *:j2 E *:i3 E *:j3"
     />
-or at any distance you like.
+or at any other distance.
-Now here is the interesting question. Again we find ourselves in the situation that trying to check that something is a 3rd row ladder escape template involves checking that infinitely many positions are edge templates. How do we get round to this?
+=== Characterization of ladder escapes ===
-Proposition: If we have a pattern plus three plussed hexes with nothing to the left of the plusses, and if the pattern becomes an edge template (connecting piece numbered 1 to the edge) when we attach each of the following two patterns to it at the plusses:
+Just like for second row ladder escapes, we again find ourselves in the situation that trying to use the definition to check that something is a 3rd row ladder escape involves checking that infinitely many positions are virtual connections. Once again, we have a theorem that allows us to replace this by a finite condition.
-<hexboard size="3x2"
+'''Theorem 2.''' Consider a candidate P for a 3rd row ladder escape. Assume that (a) L2+↑+P is a virtual connection and (b) L3+P is a virtual connection, each from the ladder stone to the bottom edge. Then P is a valid third row ladder escape.
+'''Proof.''' First note that by Theorem 1, because L2+↑+P is a virtual connection, P escapes all 2nd row ladders. Now under the assumptions of the theorem, we must show that L3+''n''+P is a virtual connection for all ''n'' ≥ 0. We prove this by induction on ''n''. For ''n'' = 0, the claim is true by assumption (b). Now suppose the claim is true for some ''n''. To show the claim for ''n''+1, consider the position L3+''n''+1+P. The first three columns of this position look like this:
+<hexboard size="3x3"
     coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
     contents="E *:a1 R 1:b1 E *:a2"
     />
-and
+(This is followed by ''n'' more columns of three empty hexes and by the pattern P). Blue has three possible moves in a triangle under stone 1, and Blue needs to play one of these or he will lose instantly. We analyze all three moves in turn.
-<hexboard size="3x1"
+For the first, Red pushes the ladder and will connect to the edge because by induction hypothesis, L3+''n''+P connects to the edge, so stone 3 connects to the edge, and so stone 1 does too.
+<hexboard size="3x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="E *:a1 R 1:b1 R 3:c1 E *:a2 B 2:b2"
+   />
+For the second, Red just wins outright, i.e., we do not need to use the induction hypothesis.
+<hexboard size="3x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="E *:a1 R 1:b1 E *:a2 R 3:c2 B 2:a3"
+   />
+And for the third, Red responds like this. Since stone 3 is a 2nd row ladder stone, it is connected to the edge because, as we noted above, ↑+P is a 2nd row ladder escape. Therefore stone 1 is also connected.
+<hexboard size="3x3"
     coords="hide"
-    contents="R 1:a2"
+   edges="bottom"
+   visible="-a1 a2"
+    contents="R 1:b1 B 2:b3 R 3:c2"
     />
-then the pattern is a third row ladder escape.
-Proof: Call the positions above A and B (so A is a 3rd row ladder and B is a second row ladder with an empty space on top). If n>=0 is an integer then by A+n I mean the position A at a distance n, that is, A followed by n columns of three empty hexes. Similarly by B+n I mean position B followed by n columns of three empty hexes. I claim if inserting A and B both result in edge templates, then inserting A+n and B+n result in edge templates for all n. The proof is by induction on n. By assumption it is true for n=0. Say we have established it for n=m>=0 (that is, we know that A+m+pattern and B+m+pattern are both edge templates) and we want to establish these facts for A+(m+1) and B+(m+1). Let us first consider the left most two columns in position B+(m+1):
+The induction is now complete, showing that P is a 3rd row ladder escape. □
-<hexboard size="3x2"
+Remark: in more concrete terms, Theorem 2 states that a pattern P is a 3rd row ladder escape if the pattern becomes a virtual connection (from the ladder stone to the edge) when we attach each of the following two patterns to its left boundary:
+A:
+<hexboard size="3x3"
     coords="hide"
-    contents="R 1:a2"
+   edges="bottom"
+   visible="-a1 a2"
+    contents="R 1:b1 E -:(c1--c3)"
     />
-(and imagine that this goes on to connect to the pattern). It is blue's move and blue only has one move which does not lose instantly. If blue plays this move then red can reply thus:
+B:
 <hexboard size="3x2"
     coords="hide"
-    contents="R 1:a2 R 3:b2 B 2:a3"
+   edges="bottom"
+   visible="-a1"
+    contents="R 1:a2 E -:(b1--b3)"
     />
-Now by our inductive hypothesis, position B+m is an edge template. This implies that piece 3 is connected to the edge, and hence piece 1 is too. So B+(m+1) is also an edge template.
-For position A+(m+1) we need to work a little more. The position we need to consider is this -- the first three columns of position A+(m+1)
+In contrast to the situation with 2nd row ladders, while Theorem 2 is ''sufficient'' to show that a position is a 3rd row ladder escape, it is not ''necessary''. For example, consider the following third row ladder escape template P:
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-d1"
+   contents="R b1 E +:a2 E +:a3 E +:a4"
+   />
+One can check directly that L3+2+P and L2+↑+2+P are both virtual connections, so that 2+P is a 3rd row ladder escape by Theorem 2. In particular, L3+''n''+P is a virtual connection for all ''n'' ≥ 2. Moreover, one can check that L3+P and L3+1+P are also virtual connections, so that P is a valid 3rd row ladder escape.
-<hexboard size="3x3"
+It is, however, not a valid 2nd row ladder escape for ladders at distance 0, because in the position L2+↑+P,
+<hexboard size="4x4"
     coords="hide"
-    contents="E *:a1 R 1:b1 E *:a2"
+   edges="bottom"
+   visible="-d1 a2"
+    contents="R b1 R 1:a3"
     />
-(and imagine that it goes on to connect to the pattern). This time blue has three moves in a triangle under piece 1 and blue needs to play one of them or blue will lose instantly. We analyse all three in turn:
+the ladder stone marked "1" cannot connect to the edge.
+Theorem 2 is therefore not sufficient to check that a given pattern is a 3rd row ladder escape. We need to work a little harder to get a necessary and sufficient condition for 3rd row ladder escapes.
+'''Lemma 3 (2nd to 3rd row jump).''' Any 3rd row ladder escape also escapes 2nd row ladders that start at distance 2 or greater. More specifically, if L3+P is a virtual connection, then so is L2+↑+2+P.
-For the first
+The lemma is perhaps easier understood in pictures: given any 3rd row ladder escape, replacing the three cells marked "+"
 <hexboard size="3x3"
     coords="hide"
-    contents="E *:a1 R 1:b1 R 3:c1 E *:a2 B 2:b2"
+   edges="bottom"
-   />
+   visible="c1 c2 c3"
-red will connect to the edge because by induction A+m connects to the edge, so piece three connects to the edge, so piece 1 does too. For the second
+    contents="E *:a1 E *:a2 E *:a3 E *:b1 E *:b2 E *:b3 E +:c1 E +:c2 E +:c3"/>
+by the pattern
 <hexboard size="3x3"
     coords="hide"
-    contents="E *:a1 R 1:b1 E *:a2 R 3:c2 B 2:a3"
+   edges="bottom"
-   />
+   visible="-a1"
-red just wins outright. And for the third this sequence is forced for blue:
+    contents="E *:a1 E +:a2 E +:a3"/>
+yields a 2nd row ladder escape.
+'''Proof.''' Assume L3+P is a virtual connection. We must show that L2+↑+2+P is a virtual connection. It looks as follows, with P added to it:
 <hexboard size="3x3"
     coords="hide"
-    contents="E *:a1 R 1:b1 E *:a2 R 3:b2 R 5:c2 B 4:a3 B 2:b3"
+   edges="bottom"
-   />
+   visible="-a1"
-and red is connected because by the inductive hypothesis B+m connects to the edge, which means that piece 5 connects to the edge, so piece 1 does too.
+    contents="E *:a1 R 1:a2"/>
+But Blue must play 2, and Red can jump to 3. Then 3 is a 3rd row ladder stone, and is connected to the edge because L3+P is a virtual connection by assumption. Therefore, 1 is also connected.
+<hexboard size="3x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1"
+   contents="E *:a1 R 1:a2 B 2:a3 R 3:c1"/>
+□
-The induction is complete, and in particular we can conclude that A+n+pattern connects to the edge for all n, so in particular the pattern is a 3rd row ladder escape.
+We now finally get a necessary and sufficient condition for 3rd row ladder escapes in the following theorem.
-QED
+'''Theorem 4.''' Consider a candidate P for a 3rd row ladder escape. Then P is a valid 3rd row ladder escape if and only if L3+P, L3+1+P, and L3+2+P are virtual connections.
-Here are some remarks about this argument.
+'''Proof.''' The left-to-right implication is trivial, since by definition, if P is valid then L3+''n''+P is a virtual connection for all ''n'', including ''n'' = 0, 1, 2. For the opposite implication, assume that L3+P, L3+1+P, and L3+2+P are virtual connections. By Lemma 3, L2+↑+2+P is a virtual connection. By Theorem 2 and the assumption about L3+2+P, 2+P is a 3rd row ladder escape. It follows that L3+''n''+P is a virtual connection for all ''n'' ≥ 2. Since we additionally assumed this to be the case for ''n'' = 0 and ''n'' = 1, P is a valid third row ladder escape. □
-(I will elaborate on these later)
+As a matter of fact, Theorem 4 is not tight. We can get the following better result. However, the proof of Theorem 4 generalizes more easily to 4th row and higher ladders, which is why it is of interest.
-) necessary but not sufficient (weird example of 3rd row escape for which hypothesis of lemma doesn't hold)
-) 3rd row escape is not a 2nd row escape
-) Is a 3rd row escape a 2nd row escape if we demand an extra row somehow? Not sure this is interesting.
-Examples: crib from King website. Fix the one where the plusses slope the wrong way.
+'''Theorem 5.''' Consider a candidate P for a 3rd row ladder escape. Then P is a valid 3rd row ladder escape if and only if L3+P and L3+1+P are virtual connections.
-== Fourth row ladder escapes ==
+'''Proof.''' The left-to-right implication is trivial. For the right-to-left implication, by Theorem 4, it suffices to show that L3+2+P is a virtual connection. Indeed, consider Blue's options in the position L3+2+P:
+<hexboard size="3x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="R 1:b1"/>
+As usual, there are only two possible moves for Blue to avoid losing immediately. If Blue moves at 2, then Red can respond at 3, which connects to the edge because L3+1+P is a virtual connection by assumption.
+<hexboard size="3x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="E *:a1 E *:a2 R 1:b1 B 2:b2 R 3:c1"/>
+If Blue instead moves at 2, then Red responds as follows, which connects to the edge because L3+P is a virtual connection by assumption.
+<hexboard size="3x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="E *:a1 E *:a2 R 1:b1 B 2:b3 R 3:b2 B 4:a3 R 5:d1"/>
+□
-We have seen all the ideas now. If you have worked through the ideas in the second and third row escapes then this will be relatively easy, other than the actual hex, which this time is quite fun!
+=== Examples ===
-A fourth row ladder escape is the following data. It is a pattern, plus four plussed hexes (not part of the pattern) going up and left from the first to the 4th row, such that there is no hex in the pattern to the left of any plussed hex. This data has to satisfy the following axiom: adding the pattern to a 4th row ladder (which I carefully defined above) at distance n, for any n>=0, gives an edge template.
+Here are some examples of third row ladder escapes. Again most of these are taken from [http://www.drking.org.uk/hexagons/hex/templates.html David King's website]. For several of the ladder escape templates, the version shown on David King's website is not minimal by our definition; in these cases, we have moved the cells marked "+" to the right to make the template minimal. All of the templates in this section have been proven to be third row ladder escapes using Theorem 5. All of them are minimal. As before, a stone marked "↓" is assumed to be connected to the bottom row, but the connection is not shown. Any shaded cells are not part of the pattern and can be occupied by Blue.
-Proposition: If adding each of the following patterns to a pattern P gives an edge template:
+The following third row ladder escapes also escape 2nd row ladders at distance 0 or greater:
-A: <hexboard size="4x3"
+<hexboard size="3x2"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 E *:a3"
+   edges="bottom"
+    contents="E +:a1 E +:a2 R b2 E +:a3"
     />
-B:<hexboard size="4x4"
+<hexboard size="3x3"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 E *:a3"
+   edges="bottom"
+    contents="E +:a1 R b1 E +:a2 E +:a3"
     />
-C:<hexboard size="4x2"
+<hexboard size="3x3"
     coords="hide"
-    contents="E *:a1 E *:a2 R 1:b2 E *:a3"
+   edges="bottom"
+   visible="-b2 c2 b3 c3"
+    contents="E +:a1 R ↓:c1 E +:a2 E *:b2 E *:c2 E +:a3 E *:b3 E *:c3"
     />
-D:<hexboard size="4x1"
+<hexboard size="4x4"
     coords="hide"
-    contents="R 1:a3"
+   edges="bottom"
+   visible="-a1 b1 d2 b3 c3 d3 b4 c4 d4"
+    contents="E *:a1 E *:b1 R ↓:d1 E +:a2 E *:d2 E +:a3 E *:b3 E *:c3 E *:d3 E +:a4 E *:b4 E *:c4 E *:d4"
     />
-then P is a 4th row ladder escape.
-Remark: again we have given a criterion which can be checked in a finite time.
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 d3"
+   contents="E *:a1 E *:a2 E +:a3 E +:a4 E +:a5 E *:b1 R d1 R d2 E *:d3 R d4"
+   />
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 c1 g1 g2"
+   contents="E *:a1 E +:a3 E +:a4 E +:a5 E *:b1 E *:c1 R e1 R f3 E *:g1 E *:g2 S e3"
+   />
+<hexboard size="5x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 c1 f3 f4 f5"
+   contents="E +:a3 +:a4 +:a5 R e3 R f2"
+   />
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 d3 d4"
+   contents="E +:(a2,a3,a4) R c1 d1"
+   />
-Remark: Note that position B is A+1, and that it's crucial that we use A+1 rather than A in this argument; this is a new phenomenon which appears for 4th row ladders.
+The following third row ladder escapes also escape 2nd row ladders at distance 1 or greater (but not at distance 0):
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1"
+   contents="E *:a1 R b1 R c1 E +:a2 E +:a3 E +:a4"
+   />
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 a2 f1 g1 g2"
+   contents="R b2 E +:a3 +:a4 +:a5"
+   />
-Proof: again the same idea. We prove simultaneously that all of A+n+P and B+n+P and C+n+P and D+n+P are edge templates, by induction on n. The case n=0 is our assumption. We need to play a little hex to get our induction going.
+The following third row ladder escapes also escape 2nd row ladders at distance 2 or greater (but not at distance 0 or 1):
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-d1"
+   contents="R b1 E *:d1 E +:a2 E +:a3 E +:a4"
+   />
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-d1"
+   contents="E +:a2 E +:a3 E +:a4 R c1 E *:d1"
+   />
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-d1"
+   contents="E +:a2 E +:a3 E +:a4 R b1 R c1 E *:d1 S b2"
+   />
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-area(b3,c2,d2,d4,b4)"
+   contents="R ↓:d1 E +:a2 +:a3 +:a4"
+   />
+<hexboard size="4x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 a2 b2 d2 e2 a3 b3 d3 e3 a4 b4 d4 e4"
+   contents="R ↓:e1 E *:a2 E *:b2 E +:c2 E *:d2 E *:e2 E *:a3 E *:b3 E +:c3 E *:d3 E *:e3 E *:a4 E *:b4 E +:c4 E *:d4 E *:e4"
+   />
+<hexboard size="5x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 c1 e2 e3 e4 e5"
+   contents="E +:a3 +:a4 +:a5 E *:a1 *:b1 *:c1 *:e2 *:e3 *:e4 *:e5 R ↓:e1 S d2"
+   />
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 c1 g1 h1 h2"
+   contents="E *:a1 E +:a3 E +:a4 E +:a5 E *:b1 E *:c1 R e1 E *:g1 E *:h1 E *:h2"
+   />
+<hexboard size="5x4"
+  coords="hide"
+  edges="bottom"
+  visible="-a1"
+  contents="E +:(a3 a4 a5) R c1 d1"
+  />
+<hexboard size="5x4"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 d1 d2"
+  contents="E +:(a3 a4 a5) R b1 c1"
+  /><hexboard size="6x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1"
+   contents="E *:a1 E *:a2 E +:a4 E +:a5 E +:a6 E *:b1 R c1 R d2"
+   />
+<hexboard size="6x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1 a2"
+   contents="E *:a1 E *:b1 R c1 E *:a2 R b2 E +:a4 E +:a5 E +:a6"
+   />
+The version of this last pattern on David King's website has the cells marked "+" (he uses arrows) sloping in the other direction; the location that is shown here makes the template minimal.
-The A case is easiest: A+(m+1)+P equals B+m+P so we're done by our inductive hypothesis.
+== Fourth row ladders ==
-The second easiest case is case D:
+=== Definition of ladder ===
-<hexboard size="4x2"
+'''Definition.''' A ''fourth row ladder'' is a pattern like this:
+L4: <hexboard size="4x4"
     coords="hide"
-    contents="R 1:a3 R 3:b3 B 2:a4"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="R c1 E -:(d1--d4)"
     />
-Remember what we are doing -- this is the first two columns of position D at distance (m+1) from the pattern. From D+(m+1) blue must defend under the red ladder, and then red can play to the above position; we know by the inductive hypothesis that D+m is an edge template, and hence D+(m+1) is too.
+Again, the red stone is called the ''ladder stone'' and Red wants to connect the ladder stone to the bottom edge. We denote this pattern by L4.
-We next deal with the position C+(m+1). First note that there are only two moves which do not lose instantly for blue. We now deal with these two moves in turn. This one
+The key part of the definition is that the 6 hexes forming a triangle below the ladder stone are all vacant. Note that even filling in one of these can invalidate the ladder: even if we fill in the bottom left corner of the triangle,
-<hexboard size="4x3"
+<hexboard size="5x5"
     coords="hide"
-    contents="E *:a1 E *:a2 R 1:b2 R 3:c2 E *:a3 B 2:b3"
+   edges="bottom"
+    contents="B a1 B b1 R c1 B d1 B e1 B a2 B b2 R c2 B a3 B b3 B a4 B a5"
     />
-leads to C+m, and this
+then Blue has this move,
-<hexboard size="4x3"
+<hexboard size="5x5"
     coords="hide"
-    contents="E *:a1 E *:a2 R 1:b2 E *:a3 R 3:b3 R 5:c3 B 4:a4 B 2:b4"
+   edges="bottom"
+    contents="B a1 B b1 R c1 B d1 B e1 B a2 B b2 R c2 B a3 B b3 B 1:d3 B a4 B a5"
     />
-leads to D+m. In either case we are done by the inductive hypothesis.
+which is easily seen to stop the ladder. To establish the ladder, Red needs at a minimum those 6 vacant hexes under her ladder stone.
+=== Definition of ladder escape ===
+The definition of a 4th row ladder escape is entirely analogous to that of 2nd and 3rd row ladder escapes.
+'''Definition.''' A ''fourth row ladder escape'' is given by a pattern P that is open on the left with a boundary of this shape.
+<hexboard size="4x1"
+  coords="none"
+  edges="bottom"
+  contents="E +:(a1--a4)"
+  />
+Moreover, it must satisfy that for all ''n'' ≥ 0, L4 + ''n'' + P is a virtual connection from the Red ladder stone to the bottom edge.
+As before, a ''candidate'' is a pattern that has the correct shape, but is not (yet) known to be a valid escape.
+=== Characterization of ladder escapes ===
+We have already encountered all of the relevant ideas. If you have worked through the ideas in the second and third row escapes then this will be relatively easy, other than the actual Hex, which this time is quite fun!
+'''Theorem 6.''' Consider a candidate P for a 4th row ladder escape. If L2+↑+↑+P, L3+↑+P, L4+P, and L4+1+P are virtual connections, then P is a 4th row ladder escape.
+'''Proof.''' The idea of the proof is the same as for 3rd row ladders. First observe that by Theorems 1 and 2, since L2+↑+↑+P and L3+↑+P are virtual connections, ↑+P escapes all 3rd row ladders and ↑+↑+P escapes all 2nd row ladders. We must prove that L4+''n''+P is a virtual connection for all ''n'' ≥ 0. We proceed by induction on ''n''. The base cases ''n'' = 0 and ''n'' = 1 hold by assumption. For the induction step, assume the claim is true for ''n'' ≥ 1. We need to prove the claim for ''n''+1.
-The B case is the most fun. We consider the following position:
+Consider the position L4+''n''+1+P. It looks like this, with ''n''−1 additional columns of four vacant hexes and the pattern P attached on the right:
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 E *:a3"
+    edges="bottom"
+   visible="-a1--a3 b1 b2"
+   contents="R 1:c1"
     />
+We need to prove that the ladder stone 1 is connected to the edge.
-Imagine that this, possibly after some columns of four vacant hexes, attaches to our pattern. We need to prove that stone 1 is connected to the edge.
+The five moves marked 2 below all lose instantly to Red 3:
-The five moves marked with a blue stone all lose instantly:
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 B d1 B e1 E *:a2 E *:b2 B e2 E *:a3 B e3 B e4"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="E *:a1 E *:b1 R 1:c1 B 2:d1 B 2:e1 E *:a2 E *:b2 B 2:e2 E *:a3 B 2:e3 B 2:e4 R 3:c2"
     />
-Any of the moves marked 2 below can be answered by red 3, moving us to position B+m+P which is an edge template by the inductive hypothesis.
+The two moves marked 2 below also lose instantly:
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 R 3:d1 E *:a2 E *:b2 B 2:c2 E *:a3 B 2:b3 B 2:a4"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="E *:a1 E *:b1 R 1:c1 B 2:b3 B 2:a4 E *:a2 E *:b2 E *:a3 R 3:d2"
     />
-Move 2 below leads us to D+m:
+The move marked 2 below can be answered by Red 3, moving us to position L4+''n''+P, which is a virtual connection by the induction hypothesis.
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:c2 B 2:d2 E *:a3 R 5:c3 R 7:d3 R 9:e3 B 4:b4 B 6:c4 B 8:d4"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="E *:a1 E *:b1 R 1:c1 R 3:d1 E *:a2 E *:b2 B 2:c2 E *:a3"
     />
-Move 2 below leads us to C+(m+1) which we have already dealt with.
+Move 2 below leads us to a 2nd row ladder, which ↑+↑+P escapes, so 5 (and therefore 1) is connected to the edge:
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:c2 R 5:d2 E *:a3 B 2:c3 B 4:b4"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:c2 B 2:d2 E *:a3 R 5:c3 B 4:b4"
     />
-Move 2 below leads us to C+m (note blue 4 must be in the triangle left and below from red 3; blue can also play out the bridge between 1 and 3 but this doesn't help):
+Both moves marked 2 below lead us to a 3rd row ladder, which ↑+P escapes, so 3 (and therefore 1) is connected to the edge:
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:d2 R 5:e2 E *:a3 B 4:c3 B 2:d3"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:d2 E *:a3 B 2:c3 B 2:b4"
     />
-Move 2 below leads us to C+m:
+Move 2 below also leads to a 3rd row ladder (note Blue 4 must be in the triangle left and below from Red 3; Blue can also play out the bridge between 1 and 3 but this doesn't help):
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:d2 R 5:e2 E *:a3 B 4:d3 B 2:b4"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:d2 R 5:e2 E *:a3 B 4:c3 B 2:d3"
     />
-Move 2 below leads us to D+m:
+Move 2 below leads us to a 2nd row ladder:
 <hexboard size="4x5"
     coords="hide"
-    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:c2 B 6:d2 E *:a3 R 7:c3 R 5:d3 R 9:e3 B 4:b4 B 2:c4 B 8:d4"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+    contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:c2 E *:a3 R 5:d3 B 4:b4 B 2:c4"
     />
-The final choice for move 2 below also leads us to D+m.
+The final choice for move 2 below also gives a 2nd row ladder.
 <hexboard size="4x5"
     coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
     contents="E *:a1 E *:b1 R 1:c1 E *:a2 E *:b2 R 3:c2 E *:a3 R 5:d3 R 7:e3 B 4:b4 B 6:c4 B 2:d4"
     />
-This completes the analysis for B and hence for all four templates.
+This completes the induction, so P is a 4th row ladder escape. □
-So the induction is finished and in particular in all cases A+m+P is an edge template, meaning that P is a 4th row ladder escape.
+Remark: In more concrete terms, Theorem 6 states that P is a 4th row ladder escape if adding each of the following patterns to P gives a virtual connection.
+A: <hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+   contents="R 1:c1 E -:(d1--d4)"
+   />
+B:<hexboard size="4x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1 b2"
+   contents="R 1:c1 E -:(e1--e4)"
+   />
+C:<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1"
+   contents="R 1:b2 E -:(c1--c4)"
+   />
+D:<hexboard size="4x2"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a2"
+   contents="R 1:a3 E -:(b1--b4)"
+   />
-QED.
+Remark: Theorem 6 is analogous to Theorem 2. It gives a sufficient, but not a necessary condition for a candidate to be a 4th row ladder escape. Once again, the criterion in Theorem 6 can be checked in a finite amount of time. To get a theorem with a necessary and sufficient condition, we need another "jump lemma":
-== 5th and 6th row ladder escapes ==
+'''Lemma 7 (3rd to 4th row jump).''' Any 4th row ladder escape also escapes 3rd row ladders that start at distance 3 or greater.
+More specifically, if L4+P and L4+1+P are virtual connections, then so is L3+↑+3+P.
+'''Proof.''' Consider the position L3+↑+3+P, which looks as follows, with P added to it:
+<hexboard size="4x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1"
+   contents="R 1:b2"
+   />
+There are only two possible moves for Blue that don't lose immediately. If Blue moves at 2, then Red can respond at 3, which is a 4th row ladder stone and connects to the edge because L4+1+P is a virtual connection by assumption.
+<hexboard size="4x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1"
+   contents="E *:a1 E *:a2 E *:a3 E *:b1 R 1:b2 B 2:b3 R 3:d1"/>
+In Blue moves instead at 2 in the following diagram, then Red can respond as shown.
+<hexboard size="4x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3 b1"
+   contents="R 1:b2 B 2:b4 R 3:b3 B 4:a4 R 5:d3 B 6:c3 R 7:d1 B 8:d2 R 9:e1"/>
+Now Red's stone 9 is a 4th row ladder stone. Although the additional red stone 5 does not belong in the L4 template, this stone can only help Red. By assumption, L4+P is a virtual connection, and so stone 9, and therefore stone 1, is connected to the edge. □
+We then arrive at a necessary and sufficient condition for fourth row ladder escapes.
+'''Theorem 8.''' Given a candiate P for a 4th row ladder escape. Then P is a valid 4rd row ladder escape if and only if L4+P, L4+1+P, L4+2+P, ..., L4+6+P are virtual connections.
+'''Proof.''' The proof is similar to that of Theorem 4. Again, the left-to-right implication is trivial. For the right-to-left implication, assume that L4+P, L4+1+P, L4+2+P, ..., L4+6+P are virtual connections. By Lemma 7 applied to P and 2+P, we know that L3+↑+3+P and L3+↑+5+P are virtual connections. By Lemma 3 applied to ↑+3+P, we know that L2+↑+2+↑+3+P is a virtual connection, and therefore also L2+↑+↑+5+P, which differs from L2+↑+2+↑+3+P only in that it contains two additional empty hexes. Since L2+↑+↑+5+P, L3+↑+5+P, L4+(5+P), and L4+(6+P) are virtual connections, we know by Theorem 6 that 5+P is a valid 4th row ladder escape. Therefore, L4+''n''+P is a virtual connection for all ''n'' ≥ 5. Since we assumed this to be also true for ''n'' = 0, 1, 2, 3, 4, it follows that P is a valid 4th row ladder escape. □
+Remark: Like Theorem 4, it is likely that Theorem 8 is not tight, in the sense that there probably exists an even simpler condition that is necessary and sufficient for 4th row ladder escapes (perhaps analogous to Theorem 5). Also, in practice, Theorem 6 is often easier to check since it involves fewer conditions.
+=== Examples ===
+Here are some examples of fourth row ladder escapes. Most are taken from [http://www.drking.org.uk/hexagons/hex/templates.html David King's website]. In each case we have moved the column of "+"s as far as possible to the right to yield a minimal template. The validity of all of these escapes has been proved using Theorem 8.
+The following fourth row ladder escape templates also escape 2nd and 3rd row ladders at distance 0 or greater:
+<hexboard size="4x2"
+   coords="hide"
+   edges="bottom"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 R b1 R b3"
+   />
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c1"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 R b2 E *:c1"
+   />
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c3 c4"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 R b3 R c1 E *:c3 E *:c4"
+   />
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1"
+   contents="E +:a2 E +:a3 E +:a4 E +:a5 R b4 R c1"
+   />
+<hexboard size="5x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 e1 f1 f2"
+   contents="E *:a1 E +:a2 E +:a3 E +:a4 E +:a5 R d2 E *:e1 E *:f1 E *:f2"
+   />
+The following fourth row ladder escape templates also escape 2nd row ladders and 3rd row ladders at distance 1 and greater.
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c3 c4"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 R b1 R c1"
+   />
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c3 c4"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 R b1 R c2"
+   />
+The following fourth row ladder escape templates also escape 2nd row ladders at distance 2 and greater, as well as 3rd row ladders at distance 1 or greater. The stone marked "↓" is assumed to be connected to the bottom edge, although the connection is now shown.
+<hexboard size="5x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1"
+   contents="E *:a1 E +:a2 E +:a3 E +:a4 E +:a5 R b1 R c2"
+   />
+<hexboard size="5x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1"
+   contents="E *:a1 E +:a2 E +:a3 E +:a4 E +:a5 R c1 R c2"
+   />
+<hexboard size="5x1"
+   coords="hide"
+   edges="bottom"
+   contents="R ↓:a1 E +:a2 +:a3 +:a4 +:a5"
+   />
+The following fourth row ladder escape templates also escape 2nd row ladders at distance 1 and greater, as well as 3rd row ladders at distance 0 or greater.
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1"
+   contents="E *:a1 E +:a2 E +:a3 E +:a4 E +:a5 E *:b1 R b2 R d1"
+   />
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 b1"
+   contents="E *:a1 E +:a2 E +:a3 E +:a4 E +:a5 E *:b1 R b2 R c1"
+   />
+<hexboard size="6x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 c1"
+   contents="E +:a3 E +:a4 E +:a5 E +:a6 R c2 R e1"
+   />
+<hexboard size="6x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 c1"
+   contents="E +:a3 E +:a4 E +:a5 E +:a6 R c2 R d1"
+   />
+The following fourth row ladder escape template also escapes 2nd row ladders at distance 2 and greater, as well as 3rd row ladders at distance 0 or greater.
+<hexboard size="6x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 c1 f3"
+   contents="E *:a1 E *:a2 E +:a3 E +:a4 E +:a5 E +:a6 E *:b1 E *:c1 R e3 R f2 E *:f3"
+   />
+== Fifth row ladders ==
+=== Definition of ladder ===
+'''Definition.''' A ''fifth row ladder'' is a pattern like this:
+L5: <hexboard size="5x6"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="R e1 E -:(f1--f5)"
+   />
+As usual, the red stone is called the ''ladder stone'' and Red's goal is to connect it to the bottom edge. We denote this pattern by L5.
+Unlike in the case of 2nd, 3rd, and 4th row ladders, this time it is not sufficient for a triangle of cells below and to the right of the ladder stone to be empty. We also need three additional empty cells to the left of this triangle. This is a minimal requirement; if even one of these cells is occupied by Blue, for example like this:
+<hexboard size="6x15"
+   coords="hide"
+   edges="bottom"
+   contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 B n1 B o1 R a2 R b2 R c2 R d2 R e2 R o2 B a3 B b3 B c3 B d3 R o3 B a4 B b4 R o4 B a5 R o5 B a6 R o6"
+   />
+then Blue can block the ladder with this move:
+<hexboard size="6x15"
+   coords="hide"
+   edges="bottom"
+   contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 B n1 B o1 R a2 R b2 R c2 R d2 R e2 R o2 B a3 B b3 B c3 B d3 R o3 B a4 B b4 R o4 B a5 R o5 B a6 R o6 B 1:e5"
+   />
+The main line is complex; see for example [http://littlegolem.net/jsp/forum/topic2.jsp?forum=50&topic=669 this Little Golem discussion thread]. Many of the main lines of defense involve Blue playing an upside-down version of [[Tom's move]], for example like this:
+<hexboard size="6x15"
+   coords="hide"
+   edges="bottom"
+   contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 B n1 B o1 R a2 R b2 R c2 R d2 R e2 R o2 B a3 B b3 B c3 B d3 R o3 B a4 B b4 R o4 B a5 R o5 B a6 R o6 B 1:e5 R 2:e4 B 3:e3 R 4:f2 B 5:f3 R 6:g2 B 7:h4 E *:d5 *:f4"
+   />
+Note that Blue's 1 is connected to 5 by double threat at "*", and 7 is Tom's move upside-down, i.e., with the top line of blue stones serving as the "edge". Therefore, to establish the ladder, Red needs at minimum the specified 13 vacant hexes under the ladder stone.
+=== Definition of ladder escape ===
+The definition of a 5th row ladder escape is as expected.
+'''Definition.''' A ''fifth row ladder escape'' is a pattern P that is open on the left with boundary of this shape:
+<hexboard size="5x1"
+  coords="none"
+  edges="bottom"
+  contents="E +:(a1--a5)"
+  />
+It must satisfy the following axiom: for all ''n'' ≥ 0, L5 + ''n'' + P connects the red ladder stone to the bottom edge, with Blue to move. As usual, a ''candiate'' is such a pattern that satisfies everything except perhaps the axiom.
+=== Characterization of ladder escapes ===
+'''Theorem 9.'''
+Consider a candiate P for a fifth row ladder escape. Assume L5+P, L5+1+P, L5+2+P, L4+↑+P, L4+↑+1+P, L3+↑+↑+P, and L2+↑+↑+↑+P are all virtual connections. Then P is a 5th row ladder escape.
+'''Proof.''' The proof idea is the same as for 3rd and 4th row ladders, but there are a lot more cases to consider. First, note that by previous theorems, ↑+P escapes all 4th row ladders, ↑+↑+P escapes all 3rd row ladders, and ↑+↑+↑+P escapes all 2nd row ladders. We prove by induction on ''n'' that L5+''n''+P is a virtual connection for all ''n'' ≥ 0. The base cases ''n'' = 0, 1, 2 are true by assumption. For the induction step, assume the claim is true for ''n'' ≥ 2. We need to prove the claim for ''n''+1.
+Consider the position L5+''n''+1+P, which looks like this (followed by an additional ''n''−2 columns of five empty hexes and the pattern P):
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4"
+   />
+The eight moves marked 2 below all lose instantly to Red 3 by [[Fourth_row_edge_templates#IV-1-a|edge template IV-1-a]]:
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f1 B 2:g1 B 2:g2 B 2:h1 B 2:h2 B 2:h3 B 2:h4 B 2:h5 R 3:e2"
+   />
+The three moves marked 2 below also lose instantly by [[Fourth_row_edge_templates#IV-1-a|edge template IV-1-a]]:
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:a5 B 2:b4 B 2:c3 R 3:f2"
+   />
+The six moves marked 2 below give a 4th row ladder, which ↑+P escapes.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:b5 B 2:c4 B 2:c5 B 2:d3 B 2:d4 B 2:e3 R 3:f2"
+   />
+This leaves us with 11 more moves to consider.
+If Blue pushes the ladder by making the move marked 2 below, Red can answer 3, moving us to position L5+''n''+P, which is a virtual connection by the induction hypothesis.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e2 R 3:f1"
+   />
+Move 2 below gives a 3rd row ladder, which ↑+↑+P escapes.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f2 R 3:e2 B 4:d4 R 5:e3"
+   />
+If Blue plays move 2 below, Red can answer 3, forcing Blue to respond in the [[ziggurat]] below and to the left of stone 3. If Blue plays in any of the cells marked 4, Red plays 5 and gets a 4th row ladder, which ↑+P escapes. Blue could have also first intruded upon the bridge between 1 and 3, but this does not help. From now on, we tacitly ignore bridge intrusions that are not helpful to Blue.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f3 R 3:f2 B 4:b5 B 4:c4 B 4:c5 B 4:d3 B 4:d4 B 4:d5 B 4:e3 B 4:e4 R 5:g2"
+   />
+If, on the other hand, Blue plays 4 below, then Red gets a 2nd row ladder, which ↑+↑+↑+P escapes.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f3 R 3:f2 B 4:e5 R 5:e3 B 6:d4 R 7:e4 B 8:d5 R 9:f4"
+   />
+If Blue plays move 2 below, Red gets a 2nd row ladder, which ↑+↑+↑+P escapes.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:g3 R 3:e2 B 4:d4 R 5:f3 B 6:e5 R 7:f4"
+   />
+If Blue plays move 2 below, Red can answer 3, forcing Blue to respond in one of the hexes marked 4. Then Red can play 5 and gets a 3rd row ladder, which ↑+↑+P escapes.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e4 R 3:e2 B 4:c5 B 4:d3 B 4:d3 B 4:d4 B 4:d5 R 5:f3"
+   />
+Similarly, if Blue plays move 2 below, Red can answer 3, forcing Blue to respond in one of the hexes marked 4. Then Red can play 5 and gets a 3rd row ladder, which ↑+↑+P escapes.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f4 R 3:f2 B 4:d5 B 4:e3 B 4:e3 B 4:e4 B 4:e5 R 5:g3"
+   />
+If Blue plays move 2 below, Red can answer 3. There are only four hexes where Blue can respond without losing outright. If Blue moves in one of the three hexes marked 4, then Red gets a 3rd row ladder, which ↑+↑+P escapes.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:g4 R 3:f2 B 4:d5 B 4:e3 B 4:e4 R 5:g3 B 6:f4 R 7:h3"
+   />
+If Blue instead moves in the hex marked 4 below, then the sequence plays out slightly differently, but Red still gets a 3rd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:g4 R 3:f2 B 4:e5 R 5:d4 B 6:e3 R 7:g3 B 8:f4 R 9:h3"
+   />
+If Blue plays move 2 below, Red can answer 3. Then Blue must respond in any of the hexes marked "+", or else Blue will immediately lose to [[Edge_templates_with_one_stone#Edge_template_III1b|edge template III1b]] or a [[ziggurat]].
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:d5 R 3:f2 E +:e5 E +:f3 E +:f4 E +:f5"
+   />
+If Blue responds in one of the two hexes marked 4, Red gets a 3rd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:d5 R 3:f2 B 4:e5 B 4:f4 R 5:g3"
+   />
+If Blue instead responds with move 4 below, Red gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:d5 R 3:f2 B 4:f3 R 5:e3 B 6:d4 R 7:e4"
+   />
+If Blue instead responds with move 4 below, Red still gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:d5 R 3:f2 B 4:f5 R 5:e3 B 6:d4 R 7:f4 B 8:e5 R 9:g4"
+   />
+If Blue plays move 2 below, Red can answer 3. Then Blue must respond in one of the hexes marked "+" to avoid immediately losing to [[Edge_templates_with_one_stone#Edge_template_III1b|edge template III1b]].
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e5 R 3:f2 E +:c5 E +:d4 E +:d5 E +:e3 E +:e4 E +:f3 E +:f4 E +:f5 E +:g3 E +:g4 E +:g5"
+   />
+If Blue responds in one of the hexes marked 4 below, Red gets a 3rd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e5 R 3:f2 B 4:c5 B 4:d4 B 4:d5 B 4:e3 B 4:e4 B 4:f4 R 5:g3"
+   />
+If Blue instead responds in one of the hexes marked 4 below, Red gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e5 R 3:f2 B 4:f3 B 4:g3 R 5:e3 B 6:d4 R 7:e4 B 8:d5 R 9:f4"
+   />
+If Blue instead responds at 4, Red gets a 3rd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e5 R 3:f2 B 4:g4 R 5:d4 B 6:e3 R 7:g3 B 8:f4 R 9:h3"
+   />
+If Blue instead responds at 4, Red gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e5 R 3:f2 B 4:f5 R 5:f3 B 6:e4 R 7:g4"
+   />
+If Blue instead responds at 4, Red gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:e5 R 3:f2 B 4:g5 R 5:f3 B 6:e4 R 7:g4 B 8:f5 R 9:h4"
+   />
+If Blue plays move 2 below, Red can answer 3. Then Blue must respond in one of the hexes marked "+" to avoid immediately losing to [[Edge_templates_with_one_stone#Edge_template_III1b|edge template III1b]] or a [[bridge]].
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f5 R 3:e2 E +:c5 E +:d3 E +:d4 E +:d5"
+   />
+If Blue responds in one of the hexes marked 4 below, Red gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f5 R 3:e2 B 4:c5 B 4:d3 B 4:d4 R 5:f3 B 6:e4 R 7:g4"
+   />
+If Blue instead responds at 4 below, Red also gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:f5 R 3:e2 B 4:d5 R 5:e3 B 6:d4 R 7:f4 B 8:e5 R 9:g4"
+   />
+Finally, if Blue plays move 2 below, Red gets a 2nd row ladder.
+<hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="E *:a1 E *:b1 E *:c1 E *:d1 R 1:e1 E *:a2 E *:b2 E *:c2 E *:d2 E *:a3 E *:b3 E *:a4 B 2:g5 R 3:e2 B 4:d4 R 5:f3 B 6:e5 R 7:g4 B 8:f5 R 9:h4"
+   />
+This completes the induction, so P is a 5th row ladder escape. □
+Remark: In more concrete terms, Theorem 9 states that P is a 5th row ladder escape if adding each of the following patterns to P gives a virtual connection.
+A: <hexboard size="5x6"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="R 1:e1 E -:(f1--f5)"
+   />
+B: <hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="R 1:e1 E -:(g1--g5)"
+   />
+C: <hexboard size="5x8"
+   coords="hide"
+   edges="bottom"
+   visible="-area(d1,a1,a4)-d2"
+   contents="R 1:e1 E -:(h1--h5)"
+   />
+D: <hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1--b3 c1"
+   contents="R 1:c2 E -:(d1--d5)"
+   />
+E: <hexboard size="5x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1--b3 c1"
+   contents="R 1:c2 E -:(e1--e5)"
+   />
+F: <hexboard size="5x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1--b2"
+   contents="R 1:b3 E -:(c1--c5)"
+   />
+G: <hexboard size="5x2"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a3"
+   contents="R 1:a4 E -:(b1--b5)"
+   />
+Like Theorems 2 and 5, Theorem 9 gives a sufficient, but not necessary condition for 5th row ladder escapes. We do not currently have a necessary and sufficient condition. One problem is that we have no appropriate "jump lemma" from 4th to 5th row ladders. In fact, we can prove that no such jump lemma is possible.
+'''Lemma 10 (No jumping from 4th to 5th row).'''  Suppose Red is the attacker in a 4th row ladder. Given enough Blue pieces on the 6th row, and enough space on the right, jumping is not an option for Red. If Red tries to jump, Blue can block the ladder, and Red will get at most a 2nd row ladder in the opposite direction.
+'''Proof.''' If Red tries to jump, Blue can play as follows.
+<hexboard size="6x13"
+   coords="hide"
+   edges="bottom"
+   contents="R a1 a2 a3 B a4 R 1:b3 B 2:b4 R 3:c3 B 4:c4 R 5:e2 B 6:e5
+             B b1 c1 d1 e1 f1 g1 h1 i1 j1 k1 l1 m1 E *:m3 *:m4 *:m5 *:m6 *:l4 *:l5 *:l6 *:k5 *:k6 *:j5 *:j6 *:i6 *:h6"
+   />
+This is exactly an upside-down version of the situation in Theorem 16 below. No matter where Red plays next, Blue can prevent Red from connecting. The hexes marked "*" are not required by Blue (i.e., they could be occupied by Red). Under [[optimal play]], Red gets at most a 2nd row ladder in the opposite direction as follows:
+<hexboard size="6x13"
+   coords="hide"
+   edges="bottom"
+   contents="R a1 a2 a3 B a4 R 1:b3 B 2:b4 R 3:c3 B 4:c4 R 5:e2 B 6:e5
+             B b1 c1 d1 e1 f1 g1 h1 i1 j1 k1 l1 m1 E *:m3 *:m4 *:m5 *:m6 *:l4 *:l5 *:l6 *:k5 *:k6 *:j5 *:j6 *:i6 *:h6
+             R 7:e3 B 8:e4 R 9:d4 B 10:c6 R 11:c5"
+   />
+See the proof of Theorem 16 for a detailed discussion of all the possible moves.  □
+Lemma 10 is a significant obstacle to establishing a necessary and sufficient criterion for 5th row ladder escapes. We do have the following generalization of Theorem 9, which gives a weaker sufficient condition (it is perhaps also necessary, but this has not been shown):
+'''Theorem 11.''' Given a candiate P for a 5th row ladder escape. If there is some ''n'' ≥ 0 such that L5+P, L5+1+P, ..., L5+''n''+P, L5+''n''+1+P, L5+''n''+2+P, as well as L4+↑+''n''+P, L4+↑+''n''+1+P, L3+↑+↑+''n''+P and L2+↑+↑+↑+''n''+P, are virtual connections, then P is a valid 5th row ladder escape.
+'''Proof.''' This follows directly from Theorem 9 applied to ''n''+P. □
+=== Examples ===
+Here are some examples of fifth row ladder escapes. The validity of these escapes has been proved using Theorem 11. These escapes are minimal.
+The following fifth row ladder escapes also escape 2nd to 4th row ladders at distance 0 or greater:
+<hexboard size="5x2"
+   coords="hide"
+   edges="bottom"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 E +:a5 R b2 R b4"
+   />
+<hexboard size="5x3"
+   coords="hide"
+   edges="bottom"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 E +:a5 R c1 R b3"
+   />
+<hexboard size="5x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c4 c5"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 E +:a5 R c2 R b4 E *:c4 *:c5"
+   />
+<hexboard size="5x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c4 c5"
+   contents="E +:(a1--a5) R b1 R b4"
+   />
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 E +:a5 R c1 R b4"
+   />
+The following fifth row ladder escape also escapes 2nd row ladders at distance 1 or greater, and 3rd and 4th row ladders at distance 0 or greater:
+<hexboard size="5x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c1 c5"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 E +:a5 R b2 R c3 E *:c1 E *:d1 E *:d2"
+   />
+The following fifth row ladder escape also escapes 2nd row ladders at distance 2 or greater, 3rd row ladders at distance 0 or greater, and 4th row ladders at distance 1 or greater:
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-c1 d1 d2"
+   contents="E +:a1 E +:a2 E +:a3 E +:a4 E +:a5 R b1 R b2 E *:c1 E *:d1 E *:d2"
+   />
+The following fifth row ladder escape also escapes 2nd row ladders at distance 2 or greater, 3rd row ladders at distance 0 or greater, and 4th row ladders at distance 2 or greater:
+<hexboard size="6x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a2--a6 e1"
+   contents="E *:a2 E *:a3 E *:a4 E *:a5 E *:a6 E +:b2 E +:b3 E +:b4 E +:b5 E +:b6 R c3 E *:e1"
+   />
+== Sixth row ladders and up ==
+Because of the [[Switchback#2nd-to-6th_row_switchback|2nd-to-6th row switchback]] trick, 6th and higher row ladders do not exist in the usual sense. More specifically, even if we allow an arbitrary amount of empty space under the ladder stone, it is not possible for the attacker to keep pushing the ladder. Consider the following situation:
+<hexboard size="7x13"
+  coords="show"
+  edges="bottom"
+  contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 R m2 R m3 R m4 R m5 R m6 R m7 R a2 B a3 R b2 B b3 R 1:c2"
+  />
+Let's assume there is an arbitrary amount of empty space in the bottom 4 rows to the left of this diagram. The stone marked "1" is connected to the top, and looks like it could be the ladder stone for a potential 6th row ladder. If such a ladder were possible, the red stones on the M-file should certainly escape it.
+From Blue's point of view, Blue is the attacker in an upside-down 2nd row ladder. Blue can therefore use an upside-down version of the [[Switchback#2nd-to-6th_row_switchback|2nd-to-6th row switchback]] trick. To do so, Blue plays at 2:
+<hexboard size="7x13"
+  coords="show"
+  edges="bottom"
+  contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 R m2 R m3 R m4 R m5 R m6 R m7 R a2 B a3 R b2 B b3 R 1:c2 B 2:e5"
+  />
+If both Red and Blue keep playing [[optimal play|optimally]], the best that Red can get is a pair of parallel 2nd and 4th row ladders in the opposite direction:
+<hexboard size="7x13"
+  coords="show"
+  edges="bottom"
+  contents="R a1 B b1 B c1 B d1 B e1 B f1 B g1 B h1 B i1 B j1 B k1 B l1 B m1 R m2 R m3 R m4 R m5 R m6 R m7 R a2 B a3 R b2 B b3 R 1:c2 B 2:e5 R 3:d3 B 4:c5 R 5:c4 B 6:b5 R 7:e4 B 8:e3 R 9:d4 B 10:e6 R 11:d5 B 12:c7 R 13:c6 B 14:b7"
+  />
+Here, stone 10 is [[virtual connection|connected]] to the line of blue stones along the top, so Red has no way of connecting right. Red can now push a 4th row ladder from 5, and/or a 2nd row ladder from 13. There is not enough space for Red to immediately perform [[Tom's move]]. So unless Red has a ladder escape somewhere to the left of this diagram, or unless there's enough space on the 5th row somewhere to the left of this diagram to perform Tom's move, Red fails to connect to the edge.
+Note that this argument does not show that 6th row ladders are categorically impossible. It only shows that the "usual" notion of ladder does not work. It is conceivable that 6th row ladders are possible under additional assumptions. For example, there might be a notion of 6th row ladder that requires additional space on the 7th row to its right, or on the 5th row to its left. It is currently unknown whether any viable notion of 6th row ladder exists.
+For 7th row ladders the situation is even worse. As explained in [[open problems about edge templates]], no amount of space under the ladder (even if we demand that the entire 5th row is clear) is known to guarantee a red connection if Blue just ignores the ladder and plays elsewhere. Thus, it is possible that 7th row ladders do not even exist in theory. Of course they do not occur in practice either.
+== Second-to-fourth row switchbacks ==
+=== Definition of switchback ===
+Informally, a 2nd-to-4th row [[switchback]] is a pattern that allows the attacker to turn around a 2nd row ladder into a ladder on the 4th row in the opposite direction. For example, in the following situation, suppose ladder stone marked "1" is connected to the top, with Blue to move.
+<hexboard size="4x7"
+    coords="show"
+    edges="bottom"
+    contents="R b1 a2 a3 1:b3 B b2 a4 R g1"
+    />
+Red pushes the 2nd row ladder to d3, the breaks at f3:
+<hexboard size="4x7"
+    coords="show"
+    edges="bottom"
+    contents="R b1 a2 a3 1:b3 B b2 a4 R g1 B 2:b4 R 3:c3 B 4:c4 R 5:d3 B 6:d4 R 7:f3"
+    />
+At this point, Blue is forced to play 8, and then a new ladder starts in the opposite direction:
+<hexboard size="4x7"
+    coords="show"
+    edges="bottom"
+    contents="R b1 a2 a3 b3 B b2 a4 R g1 B b4 R c3 B c4 R d3 B d4 R f3 B 8:e3 R 9:f1 B 10:e2 R 11:e1 B 12:d2 R 13:d1"
+    />
+In this example, the ladder reconnects to Red's original group, although in general this does not need to be the case (even if the switchback doesn't connect, Red has just created a parallel edge 4 cells from the original edge - a large advantage for Red in any case).
+To formalize the concept of a 2nd-to-4th row switchback, consider a 2nd row ladder.
+L2: <hexboard size="2x2"
+   coords="hide"
+   edges="bottom"
+   contents="R a1 E -:(b1--b2)"
+   />
+This is the same ladder as defined in the section of second-row ladders above; only this time, Red's goal will be slightly different. To explain Red's goal, we show a slightly larger area around L2:
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 c2 c3 c4"
+   contents="R a3 E *:a1 *:a2 *:b1 *:b3 *:b4 *:c2 *:c3 *:c4 a:c1 b:b2 b:b1 E -:(b3--b4)"
+   />
+This time, Red's goal will be to do at least one of the following two things: either connect the red ladder stone to the edge, or else, occupy the cell marked "a" with a red stone that is connected to the edge, without using the cells marked "b" or any cells to their left. We refer to this as the ''switchback condition''. We also call "a" the ''switchback cell'' and "b" the ''gap cells''. With this in mind, we now give the definition of a 2nd-to-4th row switchback template.
+'''Definition.''' A ''2nd-to-4th row switchback template'' (or simply 2-to-4 switchback) is given by the following data. It is a pattern P with a left boundary of shape
+<hexboard size="4x1"
+  coords="hide"
+  edges="bottom"
+  contents="E +:(a1--a4)"
+  />
+and satisfying the following axiom: L2+↑+↑+''n''+P satisfies the switchback condition for all ''n'' ≥ 0.
+As usual, a ''candidate'' is a pattern that has the correct shape, but is not (yet) known to be a valid switchback template.
+As in previous sections, we write L2+↑+↑+''n''+P for the pattern obtained from P by moving the four hexes marked "+" to the left by ''n'' columns (leaving 4 rows of empty space), then removing the top two cells marked "+" (they are not part of the pattern) and replacing the remaining cells marked "+" by L2. Note that the switchback cell "a" and gap cells "b" are not part of L2. They are simply three cells on the board whose position is defined relative to L2. Depending on the value of ''n'', they may or may not end up being inside the pattern P.
+=== Characterization of switchbacks ===
+The following theorem gives a finite necessary and sufficient condition for a pattern to be a switchback.
+'''Theorem 12.'''  Given a candidate P for a 2-to-4 switchback. Then P is a valid 2-to-4 switchback if and only if L2+↑+↑+P satisfies the switchback condition.
+'''Proof.''' The left-to-right implication is trivial. For the opposite implication, we will show, by induction on ''n'', that L2+↑+↑+''n''+P satisfies the switchback condition for all ''n'' ≥ 0. The base case ''n'' = 0 holds by assumption. Now suppose that the claim is true for some ''n''. To show the claim for ''n''+1, consider the position L2+↑+↑+''n''+1+P. It looks as follows, with ''n'' more empty columns and P appended on the right:
+<hexboard size="4x2"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="R 1:a3"
+  />
+Here, the ladder stone is marked "1". Blue has no choice but to push the ladder, and Red also pushes:
+<hexboard size="4x2"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="R 1:a3 B 2:a4 R 3:b3"
+  />
+At this point, the induction hypothesis guarantees that Red can either connect 3 to the edge, or else that Red can occupy and connect the switchback cell "a" while keeping "b" empty:
+<hexboard size="4x4"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2 c3 c4 d2 d3 d4"
+  contents="R 1:a3 B 2:a4 R 3:b3 E a:d1 b:c2 b:c1 b:b2 b:b1"
+  />
+If 3 is connected to the edge, then so is 1, and we are done. Otherwise, "a" is connected to the edge and "b" is empty. Thus, the board looks like this, with "a" now acting as a ladder stone:
+<hexboard size="4x4"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2 c3 c4 d2 d3 d4"
+  contents="E *:a1 *:a2 R a3 B a4 R b3 R 1:d1"
+  />
+Since Red's stones on the 2nd row are already connected to the top, and 1 is connected to the bottom, Blue has no choice but to respond at 2. Then Red can play 3.
+<hexboard size="4x4"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2 c3 c4 d2 d3 d4"
+  contents="E *:a1 *:a2 R a3 B a4 R b3 R 1:d1 B 2:c2 R 3:c1"
+  />
+Now the switchback condition for L2+↑+↑+''n''+1+P is satisfied, proving the theorem. □
+=== Examples ===
+Any 2nd row ladder escape template trivially also works as a switchback template (with the location of the cells marked "+" adjusted as necessary; they may need to be moved to the left if there isn't space for the two additional "+"s in the pattern). Since such a template escapes 2nd row ladders outright, there is no need for the second part of the switchback condition.
+The following are examples of 2nd-to-4th row switchback templates that are not second row ladder escapes. They are minimal.
+<hexboard size="4x4"
+  coords="hide"
+  edges="bottom"
+  visible="-d3 d4"
+  contents="E +:a1 +:a2 +:a3 +:a4 R d2"
+  />
+<hexboard size="4x4"
+  coords="hide"
+  edges="bottom"
+  visible="-d3 d4"
+  contents="E +:a1 +:a2 +:a3 +:a4 R d1"
+  />
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--c1"
+  contents="E +:a2 +:a3 +:a4 +:a5 R e1"
+  />
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--c1"
+  contents="E +:a2 +:a3 +:a4 +:a5 R d1"
+  />
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--c1"
+  contents="E +:a2 +:a3 +:a4 +:a5 R e1 R d1 S d2"
+  />
+<hexboard size="5x7"
+  coords="hide"
+  edges="bottom"
+  visible="area(a2,a5,g5,g3,f1,d1)"
+  contents="E +:a2--a5 R f2 S d5"
+  />
+In the last two templates, the shaded hex is not part of the template, and can be occupied by Blue.
+The following template is useful for obtuse corners:
+<hexboard size="5x6"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--c1 area(d5,f5,f3)"
+  contents="E +:a2 +:a3 +:a4 +:a5 R e1 f1"
+  />
+In the following template, the stone marked "↓" is assumed to be connected to the bottom edge, but the connection is not shown. The blue stone is not technically part of the pattern; however, if this cell were empty, the pattern would already work as a 2nd row ladder escape.
+<hexboard size="4x7"
+  coords="hide"
+  edges="bottom"
+  visible="-g3 g4 f4"
+  contents="E +:a1 +:a2 +:a3 +:a4 B b4 R ↓:g2"
+  />
+== Third-to-fifth row switchbacks ==
+=== Definition of switchback ===
+The definition of 3rd-to-5th row switchbacks is similar to that of 2nd-to-4th row switchbacks.
+Consider a 3rd row ladder.
+L3: <hexboard size="3x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2"
+   contents="R b1 E -:(c1--c3)"
+   />
+We define the locations of the switchback cell "a" and gap cells "b" relative to L3:
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 d2--d5"
+   contents="R b3 E b:c1 b:c2 a:d1 -:(c3--c5)"
+   />
+Again, the ''switchback condition'' states that with Blue to move, Red can either connect the ladder stone to the edge, or else Red can occupy the switchback cell and connect it to the edge, without using the gap cells or anything to their left.
+'''Definition.''' A ''3nd-to-5th row switchback template'' (or simply 3-to-5 switchback) is given by the following data. It is a pattern P, open on the left with boundary
+<hexboard size="5x1"
+  coords="hide"
+  edges="bottom"
+  contents="E +:(a1--a5)"
+  />
+and subject to the requirement that L3+↑+↑+''n''+P satisfies the switchback condition for all ''n'' ≥ 0.
+=== Characterization of switchbacks ===
+The following theorem gives a finite necessary and sufficient condition for a pattern to be a switchback. It is analogous to the corresponding theorem for 3rd row ladders.
+'''Theorem 13.'''  Given a candidate P for a 3-to-5 switchback. Then P is a valid 3-to-5 switchback if and only if L3+↑+↑+P and L3+↑+↑+1+P satisfy the switchback condition.
+'''Proof.''' The left-to-right implication is trivial. For the opposite implication, we will show, by induction on ''n'', that L3+↑+↑+''n''+P satisfies the switchback condition for all ''n'' ≥ 0. The base cases ''n'' = 0 and ''n'' = 1 hold by assumption. Now suppose that the claim is true for ''n'' and ''n''+1. To show the claim for ''n''+2, consider the position L3+↑+↑+''n''+2+P. It looks as follows, with ''n'' more empty columns and P appended on the right:
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2"
+   contents="R 1:b3"
+   />
+The ladder stone is marked "1". As usual for 3rd row ladders, Blue must either push or yield, or else Red will connect to the edge outright. If Blue pushes, then so does Red:
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2"
+   contents="R 1:b3 B 2:b4 R 3:c3"
+   />
+By induction hypothesis, L3+↑+↑+''n''+1+P satisfies the switchback condition, so either Red can connect to the edge, or else Red can get a new ladder stone 1 in the switchback cell for L3+↑+↑+''n''+1+P, which allows Red to satisfy the switchback condition for L3+↑+↑+''n''+2+P with stone 3.
+<hexboard size="5x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2"
+   contents="R b3 B b4 R c3 R 1:e1 B 2:d2 R 3:d1 E *:(c4--c5 d3--d5 e2--e5)"
+   />
+The other option is for Blue to yield. (We will see later that when ''n'' is large enough, yielding in this situation is actually a terrible idea for Blue, since it will allow Red to use P to connect to the edge. But this is not relevant for the current proof).
+<hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2"
+   contents="E *:a3 R 1:b3 E *:a4 *:a1 *:a2 *:b1 *:b2 B 2:b5 R 3:b4 B 4:a5 R 5:d3"
+   />
+Now by the induction hypothesis, L3+↑+↑+''n''+P satisfies the switchback condition, so either Red can connect to the edge, or else Red can get a new ladder stone 1 in the switchback cell for L3+↑+↑+''n''+P. This allows Red to satisfy the switchback condition for L3+↑+↑+''n''+2+P with stone 5.
+<hexboard size="5x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2"
+   contents="R b3 B b5 R b4 B a5 R d3 R 1:f1 B 2:e2 R 3:e1 B 4:d2 R 5:d1 E *:(d4--d5 e3--e5 f2--f5)"
+   />
+This finishes the proof of the theorem. □
+One may ask whether every 3-to-5 switchback template also works as a 2-to-4 switchback template. This is indeed the case at sufficient distance, due to the following jumping lemma.
+'''Lemma 14 (2nd to 3rd row switchback jump).''' Any 3-to-5 switchback template is also a 2-to-4 switchback template at distance 4 or greater. More specifically, if P is a 3-to-5 switchback template, then ↑+4+P is a 2-to-4 switchback template.
+'''Proof.''' Suppose P is a 3-to-5 switchback template, and consider Q = ↑+4+P. By Theorem 12, we must show that L2+↑+↑+Q satisfies the switchback condition. The position L2+↑+↑+Q looks like this, with P attached on the right:
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="R 1:a4"
+  />
+After Blue pushes the ladder at 2, Red plays 3, which is essentially [[Tom's move]]. While this move is not sufficient to connect Red to the edge, it creates enough trouble to allow Red to get the desired switchback in the presence of P.
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="R 1:a4 B 2:a5 R 3:d3"
+  />
+Let us consider Blue's options. If Blue moves outside the area marked "x", Red simply pushes the ladder and connects, using 3 as a ladder escape.
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="R 1:a4 B 2:a5 R 3:d3 E x:b4 x:b5 x:c4 x:c5 x:d4 x:d5 x:e3 x:e4 x:e5"
+  />
+If Blue moves in any of the cells marked 4, Red gets the switchback without using P:
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="R 1:a4 B 2:a5 R 3:d3 B 4:d4 B 4:d5 B 4:e3 B 4:e4 B 4:e5 R 5:c4 B 6:b4 R 7:c2"
+  />
+If Blue moves at 4, Red also gets the switchback without using P:
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="R 1:a4 B 2:a5 R 3:d3 B 4:b4 R 5:c2"
+  />
+If Blue moves at 4, Red also gets the switchback without using P. Note that 3 is connected by [[edge template III2b]].
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="R 1:a4 B 2:a5 R 3:d3 B 4:b5 R 5:c4 B 6:b4 R 7:c2"
+  />
+If Blue moves at 4, Red also gets the switchback without using P. Note that 3 is connected to the edge by double threat.
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="R 1:a4 B 2:a5 R 3:d3 B 4:c5 R 5:b5 B 6:b4 R 7:c2"
+  />
+This leaves only one option for Blue. If Blue moves at 4, then Red responds as follows:
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="E *:a1 *:a2 *:a3 R 1:a4 B 2:a5 R 3:d3 B 4:c4 R 5:b4 B 6:b5 R 7:d2"
+  />
+By hypothesis, since P is a 3-to-5 switchback template, Red can either connect 3 to the edge, or else get a connected red stone at "a", with "b" empty:
+<hexboard size="5x6"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a3"
+  contents="E *:a1 *:a2 *:a3 R 1:a4 B 2:a5 R 3:d3 B 4:c4 R 5:b4 B 6:b5 R 7:d2 E a:f1 b:e1 b:e2"
+  />
+In either case, 7 is connected to the edge, so Red has the desired switchback. □
+'''Corollary 15.''' In a 3rd row ladder at distance 5 or greater to a 3-to-5 switchback, Blue cannot yield.
+'''Proof.''' If Blue yields, then Red can switch back the resulting 2nd row ladder to the 4th row by the previous lemma. This will reconnect to Red's original 3rd row ladder, and therefore connect Red to the edge. In a diagram:
+<hexboard size="5x7"
+  coords="hide"
+  edges="bottom"
+  visible="-a1--a4 b1 b2"
+  contents="R 1:b3 B 2:b5 R 3:b4 B 4:a5 R 5:e3 B 6:d4 R 7:c4 B 8:c5 R 9:e2 E a:g1 b:f1 b:f2"
+  />
+Note that Blue must play 6 for the same reason as in the lemma. Since Red will either connect 5 or "a" to the edge, 7 is also connected. Rather than just giving Red a switchback, 7 is actually connected to 1 by a [[Interior template#The crescent|crescent]]. □
+Here is another interesting fact about 3-to-5 switchbacks. Given enough space, the defender of a 3rd row ladder cannot yield without giving the attacker a switchback.
+'''Theorem 16.''' Given enough space to the right of a 3rd row ladder and two empty rows above it, if the defender tries to yield, the attacker can achieve a 3-to-5 switchback without requiring any addtional stones.
+'''Proof.''' Let 1 be the ladder stone of a 3rd row ladder, and assume there is at least as much space as indicated in the following diagram.
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R 1:b3"
+   />
+If Blue yields at 2, then Red can play as follows:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R 1:b3 B 2:b5 R 3:b4 B 4:a5 R 5:e2
+   E x:c2 x:c3 x:c4 x:c5 x:d2 x:d3 x:d4 x:d5 x:e3"
+   />
+If Blue's next move is outside of the area marked "x", then Red connects to the edge outright, as follows:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   R 7:d4 B 8:c4 R 9:d2"
+   />
+Therefore, Blue must move in one of the hexes marked "x" above. This leaves nine possible moves for Blue.
+If Blue moves at one of the hexes marked 6 below, then Red connects by [[edge template IV2b]]:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:c2 6:d2 R 7:d3"
+   />
+If Blue moves at 6, then Red pushes the second row ladder twice and connects by [[Tom's move]]:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:c3 R 7:c4 B 8:c5 R 9:d4 B 10:d5 R 11:g3"
+   />
+If Blue moves at 6, then Red gets 2nd and 4th row parallel ladders, which connect by [[Tom's move]]:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:d3 R 7:d2 B 8:e3 R 9:c4 B 10:c5 R 11:d4 B 12:d5 R 13:g3"
+   />
+If Blue moves at 6, then Red connects by a [[Interior template#The crescent|crescent]] and [[Tom's move]]:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:e3 R 7:c4 B 8:c5 R 9:d4 B 10:d5 R 11:g3"
+   />
+If Blue moves at 6, then Red connects by [[Interior template#The crescent|crescent]] and [[ziggurat]]:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:d4 R 7:c4 B 8:c5 R 9:f3"
+   />
+If Blue moves at 6, then Red connects by [[Interior template#The shopping cart|shopping cart]] and [[Tom's move]]:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:c5 R 7:d4 B 8:d5 R 9:g3"
+   />
+If Blue moves at 6, the situation is almost identical:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:d5 R 7:d4 B 8:c5 R 9:g3"
+   />
+Note that in all cases so far, Red connected outright, i.e., didn't need a switchback. The final remaining possibility is for Blue to move at 6 in the following diagram. Then Red gets the switchback. Note that 7 is connected to the edge by [[edge template IV2b]].
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   visible="-a1--a4 b1 b2 i1 j1 j2"
+   contents="E *:a1 *:a2 *:a3 *:a4 *:b1 *:b2 *:i1 *:j1 *:j2 R b3 B b5 R b4 B a5 R e2
+   B 6:c4 R 7:d3 B 8:c3 R 9:d1"
+   />
+This finishes the proof of the theorem. □
+=== Examples ===
+Every 3rd row ladder escape template is also a 3-to-5 switchback template (possibly with the location of the column of "+"s adjusted), but it need not be minimal. Here are some examples of 3-to-5 switchback templates that are not 3rd row ladder escapes.
+<hexboard size="5x5"
+  coords="hide"
+  edges="bottom"
+  visible="-e1"
+  contents="E +:a1 +:a2 +:a3 +:a4 +:a5 *:e1 R e3"
+  />
+<hexboard size="5x4"
+  coords="hide"
+  edges="bottom"
+  contents="E +:a1 +:a2 +:a3 +:a4 +:a5 R d2"
+  />
+<hexboard size="5x4"
+  coords="hide"
+  edges="bottom"
+  contents="E +:a1 +:a2 +:a3 +:a4 +:a5 R d1"
+  />
+<hexboard size="5x4"
+  coords="hide"
+  edges="bottom"
+  contents="E +:a1 +:a2 +:a3 +:a4 +:a5 R d1 R c1 S c2"
+  />
+The shaded cell is not part of the template and can be occupied by Blue.
+== Second and fourth row parallel ladders ==
+=== Definition of ladder ===
+[[Parallel ladder]]s, especially on the 2nd and 4th rows, are quite common in Hex. For example, consider this situation, with Blue to move and the Red stone connected to the top:
+<hexboard size="4x7"
+  coords="hide"
+  edges="bottom"
+  contents="B b2 R d1"
+  />
+Play may proceed as follows:
+<hexboard size="4x7"
+  coords="hide"
+  edges="bottom"
+  contents="B b2 R d1 B 1:d2 R 2:e1 B 3:e2 R 4:c2 B 5:b4 R 6:c3 B 7:c4"
+  />
+Now Red has a choice: she can either continue pushing the 4th row ladder from 2, or the 2nd row ladder from 6. However, having parallel ladders puts Red in a stronger position than having a 2nd row ladder or a 4th row ladder alone. As we will see, there exist ladder escape templates than can escape a parallel ladder, but can neither escape a 2nd row ladder nor a 4th row ladder on its own.
+'''Note.''' Unlike with single-row ladders, in the case of a parallel ladder, Red actually has a choice whether to push the 2nd row ladder or the 4th row ladder. For this reason, our formal definition of a parallel ladder follows a slightly different approach than that we took for single-row ladders above. Whereas above, we always assumed that ''Blue'' was next to move (and the ladder stone was already in a pushing position), here, we will assume that ''Red'' is next to move. This affects the definition of the ladder pattern, in that the ladder stones do not yet have empty space below them.
+'''Definition.''' A ''parallel ladder on the 2nd and 4th rows'', or ''2-4 parallel ladder'' for short, is a pattern like this:
+L24: <hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 E -:(d1--d4)"
+   />
+The red stones on the second and fourth rows are called the ladder stones, and Red's goal is to connect at least one of these stones to the bottom edge, with ''Red'' to move first. (We can assume that both ladder stones are already connected to the top). We denote this pattern by L24. There is also a variant of L24 that looks like this:
+L24a: <hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 a3"
+   contents="R b2 R c1 B a4 c2 E -:(d1--d4)"
+   />
+L24 and L24a are equivalent, and for simplicity we will only use L24.
+=== Definition of ladder escape ===
+'''Definition.''' An ''escape template for parallel ladders on the 2nd and 4th rows'', or ''2-4 parallel ladder escape'' for short, is a pattern P with a left boundary of shape
+<hexboard size="4x1"
+  coords="hide"
+  edges="bottom"
+  contents="+:(a1--a4)"
+  />
+satisfying the following axiom: adding a 2-4 parallel ladder at distance ''n'', for any ''n'' ≥ 0, allows Red to connect, in the sense that it guarantees the connection of at least one of the red ladder stones to the bottom edge, with Red to move.
+As before, a ''candidate'' is a pattern that has the correct shape, but is not (yet) known to be a valid escape.
+=== Characterization of ladder escapes ===
+Fortunately, 2-4 parallel ladders are easy to analyze; they are almost as simple as 2nd row ladders. The reason is that, just as for 2nd row ladders, the defender has no choice; he must always push, because as we will see, yielding is not an option. We get a simple and clean theorem with a necessary and sufficient condition for 2-4 parallel ladder escapes.
+'''Theorem 17.''' Consider a candidate P for a 2-4 parallel ladder escape. Then P is a valid 2-4 parallel ladder escape if and only if L24+P, L24+1+P, and L24+2+P allow Red to connect (with Red to move).
+'''Proof.''' If the ladder escape is valid, then by definition, L24+''n''+P allows Red to connect for all ''n'', including ''n'' = 0, 1, 2. So the left-to-right implication is trivial. To prove the right-to-left implication, assume L24+P, L24+1+P, and L24+2+P allow Red to connect. We prove by induction that L24+''n''+P allows Red to connect for all ''n'' ≥ 0. The cases ''n'' = 0, 1, 2 are true by assumption. Now suppose the claim is true for some ''n'' ≥ 2. We must show the claim for ''n''+1. To do so, consider the position L24+''n''+1+P. The first six columns of this position look like this:
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2"
+   />
+This is followed by ''n''−2 more columns of four empty hexes and by the pattern P. Red starts by pushing the 4th row ladder.
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1"
+   />
+Now consider Blue's possible moves. If Blue moves in any of the hexes marked 2 below (or elsewhere on the board), Red wins outright (i.e., without using the induction hypothesis).
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 R 1:d1 B a4 c2 B 2:e1 2:e2 2:e3 2:e4 2:f1 2:f2 2:f3 2:f4 2:d3 2:d4 R 3:c3"
+   />
+If Blue moves in any of the hexes marked 2 below, Red also wins outright:
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 R 1:d1 B a4 c2 B 2:b3 2:b4 2:c3 R 3:e2"
+   />
+If Blue moves at 2 below, Red also wins outright:
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 R 1:d1 B a4 c2 B 2:c4 R 3:b4 B 4:b3 R 5:d2 B 6:c3 R 7:e3"
+   />
+This means that the only possible move that is not immediately losing for Blue is to push the 4th row ladder. In this case, Red can respond as follows:
+<hexboard size="4x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 R 1:d1 B a4 c2 B 2:d2 R 3:b3 B 4:b4"
+   />
+This position allows Red to connect by induction hypothesis, finishing the proof. □
+It is clear that every 2nd row ladder escape and every 4th row ladder escape is also an escape for 2nd-and-4th row parallel ladders, since Red can decide to push only the 2nd row ladder, or only the 4th row ladder. In addition, 2nd-to-4th row switchback templates also work as 2-4 parallel ladder escapes. This is intuitively clear, as Red can simply push the 2nd row ladder and switch it back to the 4th row, where it will connect with the 4th row of the parallel ladder. The following theorem proves this more formally, using the definitions.
+'''Theorem 18.''' Every 2nd-to-4th row switchback template is also a 2-4 parallel ladder escape.
+'''Proof.''' Assume P is a 2nd-to-4th row switchback template. To show that P is a 2-4 parallel ladder escape, we must show that L24+''n''+P allows Red to connect with Red to move, for all ''n'' ≥ 0. Consider the position L24+''n''+P, which looks as follows, with an additional ''n'' blank columns and P on the right:
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2"
+   />
+Red plays as follows. At this point, since ''n''+P is a 2nd-to-4th row switchback template, Red can either connect 3 to the edge, or get a connected stone at "a" with "b" empty.
+<hexboard size="4x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:b3 B 2:b4 R 3:c3 E a:e1 E b:d1 b:d2"
+   />
+This allows Red to connect at least one of the ladder stones, as required. □
+=== Examples ===
+As mentioned above, every 2nd row ladder escape, every 4th row ladder escape, and every 2nd-to-4th row switchback template works as a 2-4 parallel ladder escape. But there are some examples of 2-4 parallel ladder escapes that are none of the above. The most famous of these is [[Tom's move]], which states that a sufficient amount of empty space is enough for a 2-4 parallel ladder to connect to the edge. Specifically, the following is a 2-4 parallel ladder escape template:
+<hexboard size="5x6"
+  coords="hide"
+  edges="bottom"
+   visible="-a1 e1 f1 f2"
+  contents="E +:a2 +:a3 +:a4 +:a5"
+  />
+Other examples:
+<hexboard size="5x5"
+   visible="-a1 a2 b1 e4 e5 a3--a5 e3"
+   edges="bottom"
+   coords="none"
+   contents="E +:b2--b5 R d1 e1 B d2"
+   />
+Here are some other examples of 2-4 parallel ladder escapes that are neither 2nd nor 4th row ladder escapes nor 2nd-to-4th row switchbacks. They can be shown to be valid by Theorem 17, and are minimal. Unlike Tom's move, these ladder escapes don't require space on the 5th row.
+While the following two patterns aren't switchbacks at distance 0 or 1, they do work as 2nd-to-4th row switchbacks at distance 2 or greater.
+<hexboard size="4x2"
+  coords="hide"
+  edges="bottom"
+   visible="-b3 b4"
+  contents="E +:a1 +:a2 +:a3 +:a4 R b2"
+  />
+<hexboard size="4x2"
+  coords="hide"
+  edges="bottom"
+   visible="-b3 b4"
+  contents="E +:a1 +:a2 +:a3 +:a4 R b1"
+  />
+== Third and fifth row parallel ladders ==
+=== Definition of ladder ===
+Parallel ladders on the 3rd and 5th rows are less common than those on the 2nd and 4th rows, but they can occur. Pushing such ladders is less straightforward, as the defender has more options. Basically, as we will show, if the defender refuses to push, then the attacker can at least get a 2nd row ladder. Moreover, a 2nd-to-4th row switchback template is in that case sufficient for the attacker to connect.
+'''Definition.''' A ''parallel ladder on the 3nd and 5th rows'', or ''3-5 parallel ladder'' for short, is a pattern like this:
+L35: <hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 E -:(d1--d5)"
+   />
+The red stones on the third and fifth rows are again called the ladder stones, and Red's goal is to connect at least one of these stones to the bottom edge, with ''Red'' to move first. We denote this pattern by L35. Just like for 2-4 parallel ladders, there is an equivalent pattern for L35 that looks like this:
+L35a: <hexboard size="5x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 a3"
+   contents="R b2 R c1 B a4 c2 E -:(d1--d5)"
+   />
+=== Definition of ladder escape ===
+'''Definition.''' An ''escape template for parallel ladders on the 3rd and 5th rows'', or ''3-5 parallel ladder escape'' for short, is a pattern P with a left boundary of shape
+<hexboard size="5x1"
+  coords="hide"
+  edges="bottom"
+  contents="E +:(a1--a5)"
+  />
+satisfying the following axiom: adding a 3-5 parallel ladder at distance ''n'', for any ''n'' ≥ 0, allows Red to connect, in the sense that it guarantees the connection of at least one of the red ladder stones to the bottom edge, with Red to move.
+As always, a ''candidate'' is a pattern that has the correct shape, but is not (yet) known to be a valid escape.
+=== Characterization of ladder escapes ===
+-5 parallel ladder escapes are not quite as easy to characterize as those for 2-4 parallel ladders, because the defender has more options. We get the following theorem, which only contains a sufficient condition for a pattern to be a 3-5 parallel ladder escape.
+'''Theorem 19.''' Consider a candidate P for a 3-5 parallel ladder escape. If L35+P, L35+1+P, ..., L35+3+P allow Red to connect (with Red to move), and if ↑+P is a 2nd-to-4th row switchback template, then P is a valid 3-5 parallel ladder escape.
+'''Proof.''' We prove by induction that L35+''n''+P allows Red to connect for all ''n'' ≥ 0. The cases ''n'' = 0, ..., 3 are true by assumption. Now suppose the claim is true for 0, ..., ''n'', where ''n'' ≥ 3. We must show the claim for ''n''+1. To do so, consider the position L35+(''n''+1)+P. The position looks like this, followed by ''n''−3 additional empty columns and P:
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2"
+   />
+Red starts by pushing the 5th row ladder.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 E x:a5 x:b4 x:b5 x:c3 x:c4 x:c5 x:d2 x:d3 x:d4 x:d5 x:e1 x:e2 x:e3 x:e4 x:e5 x:f2 x:f3 x:f4 x:f5 x:g3 x:g4 x:g5"
+   />
+Now consider Blue's possible moves. If Blue moves anywhere except the hexes marked "x", then Red wins outright by bridging from 1 to [[edge template IV1a|edge template IV-1a]].
+If Blue moves in any of the hexes marked 2 below, Red moves at 3 and connects by [[ziggurat]] and [[double threat]].
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:e1 2:e2 2:e3 2:e4 2:e5 2:f2 2:f3 2:f4 2:f5 2:g3 2:g4 2:g5 R 3:c3"
+   />
+This leaves 10 more moves to consider.
+If Blue moves at 2, Red pushes the 3rd row ladder.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:d2 R 3:b3 E x:b4 y:b5"
+   />
+Now Blue must either push at "x" or yield at "y" (or else Red will connect immediately). If Blue pushes at "x", then Red has a 3-5 parallel ladder at distance ''n'', which connects by induction hypothesis.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:d2 R 3:b3 B 4:b4"
+   />
+If Blue instead yields at "y", then Red can push the 2nd row ladder and use the switchback to either connect 7 to the edge or get a connected stone at "a". Note that "a" is connected to either 1 or 7 by double threat, so Red connects. (As a matter of fact, Red can do better in this case and get a 2-4 parallel ladder, but it is not needed for the current proof).
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:d2 R 3:b3 B 4:b5 R 5:c4 B 6:c5 R 7:d4 E a:f2"
+   />
+If Blue moves at 2, then Red plays as follows and connects by [[edge template III2e]]:
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:c3 R 3:b4 B 4:b3 R 5:e2 B 6:e3 R 7:d3"
+   />
+If Blue moves at 2, then Red gets a 2nd row ladder that connects via the 2-to-4 switchback at "a". The moves 4 and 5 can also be played in the opposite order without changing the result.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:d3 R 3:c3 B 4:d2 R 5:b3 B 6:b5 R 7:c4 B 8:c5 R 9:d4 E a:f2"
+   />
+If Blue moves at 2, then Red gets a 2nd row ladder that connects via the 2-to-4 switchback at "a". (In fact, Red can get a 2-4 parallel ladder, but it is not needed in this proof).
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:b4 R 3:e2 B 4:e3 R 5:d3 B 6:c5 R 7:d4 B 8:d5 R 9:e4 E a:g2"
+   />
+If Blue moves at 2, Red connects.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:c4 R 3:b4 B 4:b3 R 5:d2 B 6:c3 R 7:e3"
+   />
+If Blue moves at 2, Red connects
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:d4 R 3:b4 B 4:b3 R 5:e2 B 6:d3 R 7:f3"
+   />
+If Blue moves at 2, Red connects by [[edge template IV2d]].
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:a5 R 3:c4 B 4:c3 R 5:e2"
+   />
+If Blue moves at 2, Red can respond at 3.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:b5 R 3:e2 E x:e3 y:d5"
+   />
+Now Blue must respond at "x" or "y", or else Red will connect immediately. If Blue plays at "x", Red gets a 2nd row ladder which connects via switchback at "a". Note that 5 is connected to at least one ladder stone by double threat.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:b5 R 3:e2 B 4:e3 R 5:c4 B 6:c5 R 7:d4 B 8:d5 R 9:e4 E a:g2"
+   />
+If Blue instead plays at "y", Red also gets a 2nd row ladder which connects via switchback at "a".
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:b5 R 3:e2 B 4:d5 R 5:d4 B 6:c5 R 7:e4 E a:g2"
+   />
+If Blue moves at 2, Red connects.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:c5 R 3:b4 B 4:b3 R 5:d2 B 6:c3 R 7:d3 B 8:c4 R 9:e4"
+   />
+Finally, if Blue moves at 2, Red also connects.
+<hexboard size="5x7"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 b2"
+   contents="R a3 R c1 B a4 c2 R 1:d1 B 2:d5 R 3:b4 B 4:b3 R 5:d2 B 6:c3 R 7:e3 B 8:f4 R 9:d4"
+   />
+This finishes the proof. □
+=== Examples ===
+Like 2-4 parallel ladders, 3-5 parallel ladders have the property that they can connect to the edge outright if given enough space. There is an analog of [[Tom's move]] for 3-5 parallel ladders. The following diagram shows the amount of space required. If Red moves in the cell marked "x", Red can guarantee to connect at least one of the ladder stones marked "1" to the edge. The cell marked "x" is essentially the unique winning move (the only other winning option for Red is to push the 3rd row ladder one more hex before playing "x").
+<hexboard size="6x12"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 a3 b1 b2 b3 c1 g1 h1 i1 j1 k1 k2 l1 l2 l3"
+   contents="R 1:a4 1:c2 E x:e3 B a5 c3"
+   />
+We note that this particular pattern is not technically a 3-5 parallel ladder escape. Without additional empty space on the 6th row, it only escapes 3-5 parallel ladders at distance 0 (as shown) and at distance 1. If the ladder starts further away, Blue has the option of yielding to a 2nd row ladder for which Red would need a 2-to-4 switchback template to connect.
+Every 3rd row ladder escape, every 5th row ladder escape, and every 3-to-5 switchback template is also a 3-5 parallel ladder escape. Examples of 3-5 parallel ladder escapes that aren't one of the above are relatively rare, but they do exist. The following are some examples. They have been proved correct using Theorem 19, and they are minimal.
+The following template escapes 3-5 parallel ladders, but it is not a 3rd-to-5th row switchback, nor does it escape 3rd, 4th, or 5th row ladders.
+<hexboard size="6x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 e1 f1 f2"
+   contents="E +:(a2--a6) R f6"
+   />
+The following template escapes 3-5 parallel ladders, but it is not a 3rd-to-5th row switchback, nor does it escape 2nd, 3rd, 4th, or 5th row ladders.
+<hexboard size="6x8"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 e1 f1 f2 g1 g2 h1 h2 h5 h6"
+   contents="E +:(a2--a6) R h4"
+   />
+== Second and fourth row terraced ladders ==
+Sometimes it can happen that a ladder forms on top of another ladder, with the two rows of attacking stones not yet connected to the edge nor to each other. We call this a ''terraced ladder''. The following is an example of a terraced ladder on the 2nd and 4th rows, with Blue to move:
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   contents="R a2 B a3 R b2 B b3 R c2 B c3 R d2  B a4 B a5 R b4 B b5 R c4 B c5 R d4 B d5 R e4 B e5 R f4"
+   />
+Although terraced ladders look superficially similar to parallel ladders, they should not be confused. There are two important differences: (1) in a parallel ladder, the two rows of attacking stones are connected to each other, whereas in a terraced ladder, they are not, and (2) in a parallel ladder, the upper ladder is "ahead" of the lower one, whereas in a terraced ladders, the upper ladder is at the same level or behind the lower ladder.
+In fact, as we noted above, from the attacker's point of view, having 2nd and 4th row parallel ladders is ''stronger'' than having only a 2nd row ladder or only a 4th row ladder. For terraced ladders, the opposite is true: a 2nd and 4th row terraced ladder is ''weaker'' than having only a 2nd row ladder or only a 4th row ladder. Nevertheless, despite being relatively weak, terraced ladders can be pushed, and there is a notion of terraced ladder escape at arbitrary distance.
+Before we develop the theory of terraced ladders, it is worth noting that terraced ladders from Red's point of view are parallel ladders from Blue's point of view, and vice versa. This can be seen by putting a row of blue stones on top, giving Blue an "edge":
+<hexboard size="5x10"
+   coords="hide"
+   edges="bottom"
+   contents="R a2 B a3 R b2 B b3 R c2 B c3 R d2 B a4 B a5 R b4 B b5 R c4 B c5 R d4 B d5 R e4 B e5 R f4 B a1 b1 c1 d1 e1 f1 g1 h1 i1 j1"
+   />
+Indeed, from Red's point of view, Red has terraced ladders trying to connect to the bottom edge, whereas from Blue's point of view, Blue has parallel ladders trying to connect to the top edge. The fact that parallel ladders are better for Blue than individual ladders explains why terraced ladders are worse for Red than individual ladders.
+=== Definition of ladder ===
+In a terraced ladder, it is the defender, not the attacker, who decides whether to push the 2nd or 4th row ladder. Since the 4th row ladder can lag behind the 2nd row ladder by an arbitrary distance, there isn't just a single ladder template, but a family of them.
+'''Definition.''' A 2nd and 4th row terraced ladder is any one of the following patterns:
+T(0):
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 c3 c4"
+   contents="R a3 b1 E -:(c1 c2 b3 b4)"
+   />
+T(1):
+<hexboard size="4x3"
+   coords="hide"
+   edges="bottom"
+   visible="-c3 c4"
+   contents="R a1 a3 E -:(c1 c2 b3 b4)"
+   />
+T(2):
+<hexboard size="4x4"
+   coords="hide"
+   edges="bottom"
+   visible="-a4 d3 d4"
+   contents="R a1 a3 R b3 E -:(d1 d2 c3 c4)"
+   />
+T(3):
+<hexboard size="4x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a4 b4 e3 e4"
+   contents="R a1 a3 R b3 R c3 E -:(e1 e2 d3 d4)"
+   />
+and so on. In general, for ''k'' ≥ 1, the pattern T(''k'') looks like
+<hexboard size="4x1"
+   float="inline"
+   coords="hide"
+   edges="bottom"
+   visible="-a4"
+   contents="R a1 a3"
+   /> followed by ''k''−1 columns of <hexboard size="4x1"
+   float="inline"
+   coords="hide"
+   edges="bottom"
+   visible="-a4"
+   contents="R a3"
+   /> followed by <hexboard size="4x3"
+   float="inline"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 a3 c3 c4"
+   contents="R a3 E -:(c1 c2 b3 b4)"
+   />
+In these patterns, we refer to Red's stone on the 4th row as the ''top ladder stone'', and to Red's rightmost stone on the 2nd row as the ''bottom ladder stone''. Red's goal is to connect the top ladder stone to the bottom edge, assuming it is Blue's turn first. We can assume that the top ladder stone is already connected to the top edge, but we do not assume that the top and bottom ladder stones are connected to each other.
+=== Definition of ladder escape ===
+'''Definition.''' A ladder escape template for 2nd and 4th row terraced ladders, or 2-4 terraced ladder escape for short, is a pattern P with left boundary shaped like this,
+<hexboard size="4x2"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2 b3 b4"
+  contents="E +:(a3 a4 b1 b2)"
+  />
+This pattern P must satisfy the following axiom: for all ''k'' ≥ 0 and all ''n'' ≥ 0, T(''k'')+''n''+P guarantees a connection of the top ladder stone to the bottom edge, with Blue to move.
+As always, a candidate is a pattern that has the correct shape, but is not (yet) known to be a valid escape. If P is such a candidate, schematically of the form
+<hexboard size="4x3"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2"
+  contents="E +:(a3 a4 b1 b2) *:b3 *:b4 *:c1 *:c2 *:c3 *:c4"
+  />
+then we write ∗+P to denote the pattern obtained from P by replacing the top two cells marked "+" by empty cells, and adding two new cells marked "+" just to their left. The resulting template is then of the shape required for 4th row ladder escapes.
+<hexboard size="4x3"
+  coords="hide"
+  edges="bottom"
+  contents="E +:(a1--a4) *:b3 *:b4 *:c1 *:c2 *:c3 *:c4"
+  />
+=== Characterization of ladder escapes ===
+We will reduce the problem of establishing a terraced ladder escape to finitely many cases. This is done by two lemmas. Lemma 20 states that we only need to consider finitely many values of ''n'' (the distance from the bottom ladder stone to the escape). Lemma 21 states that we only need to consider finitely many values of ''k'' (the distance from the top ladder stone to the bottom ladder stone). .
+'''Lemma 20.''' Suppose P is a candidate for a 2-4 terraced ladder escape. Assume T(''k'')+P, T(''k'')+1+P, and T(''k'')+2+P are virtual connections for all ''k'' ≥ 0 (i.e., they guarantee a connection of the top ladder stone to the edge, with Blue to move). Moreover, assume that ∗+P escapes 2nd, 3rd, and 4th row ladders (or more precisely, ∗+P escapes 4th row ladders, ↑+∗+P (which has 3 cells marked "+") escapes 3rd row ladders, and ↑+↑+∗+P escapes 2nd row ladders). Then P is a valid 2-4 terraced ladder escape.
+'''Proof.''' We need to show that T(''k'')+''n''+P is a virtual connection for the top ladder stone, for all ''k'',''n'' ≥ 0. We prove this by nested induction, with the outer induction being on ''n'', and the inner induction on ''k''. The base cases ''n'' = 0, 1, 2 are true by assumption. Now consider some ''n'' ≥ 3, and suppose the claim is true up to ''n''−1. We need to show the claim for ''n''. Consider the position T(''k'')+''n''+P, which looks like this:
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 E x:b1 x:c1 x:d1 x:e1 x:a2 x:b2 x:c2 x:d2 x:e2 x:f2 x:e3 x:f3 x:d4 x:e4 x:f4"
+  />
+followed by ''n''−3 additional empty columns and P. Here, our diagram illustrates the case ''k'' = 4, but the following arguments are valid for all ''k'' ≥ 0. The first observation is that if Blue's next move is outside of the area marked "x", Red connects to the edge immediately by a [[Interior_template#The_long_crescent|long crescent]] and [[edge template III2b]]:
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 R e2 "
+  />
+Note that this works for all ''k'' ≥ 0, although for ''k'' = 0 and ''k'' = 1, the connection is simpler and does not require a long crescent. Therefore, Blue must move in the area marked "x".
+We consider each of Blue's options in turn. If Blue moves just below the top ladder stone, then Red responds by pushing the 4th row ladder. In case ''k'' > 0, this leads to the position T(''k''−1)+''n''+P, and he claim holds by the inner induction hypothesis:
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 B 1:a2 R 2:b1"
+  />
+In case ''k'' = 0, the situation is worse for Blue: in this case, Red gets a bona fide 4th row ladder, which ∗+P escapes by assumption:
+<hexboard size="4x5"
+  coords="hide"
+  edges="bottom"
+  visible="-a1 a2 e3 e4"
+  contents="R a3 b1 B 1:b2 R 2:c1"
+  />
+If Blue moves anywhere else on the 3rd or 4th row, then Red connects the two ladders and gets a second row ladder, which ↑+↑+∗+P escapes by assumption. This works for all ''k'' ≥ 0, although for illustration, we show only the case ''k'' = 4:
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 B 1:b1 1:b2 1:c1 1:c2 1:d1 1:d2 1:e1 1:e2 1:f2 R 2:a2"
+  />
+This leaves only 5 possible Blue moves to consider.
+If Blue moves at 1, Red gets a 3rd row ladder, which ↑+∗+P escapes. Note that 2 is connected to the top ladder stone by a [[Interior_template#The_long_crescent|long crescent]] (for ''k'' ≥ 2) or directly (for k = 0, 1).
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 B 1:e3 R 2:e2 B 3:d4 R 4:f2"
+  />
+If Blue moves at 1, Red gets a 3rd row ladder, which ↑+∗+P escapes by assumption:
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 B 1:f3 R 2:f2 B 3:e2 R 4:a2 B 5:d4 R 6:e3 B 7:e4 R 8:g2"
+  />
+Note that there are several alternatives to Blue's move 3, but they all result in a 3rd row ladder for Red.
+If Blue moves at 1, Red simply pushes the 2nd row ladder, and we are now in position T(''k''+1)+''n''−1+P, which is a virtual connection by the outer induction hypothesis.
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 B 1:d4 R 2:e3"
+  />
+If Blue moves at 1, Red gets a 2nd row ladder, which ↑+↑+∗+P escapes by assumption. Note again that 2 is connected to the top ladder stone.
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 B 1:e4 R 2:e2 B 3:d4 R 4:f3"
+  />
+If Blue moves at 1, Red gets a 2nd row ladder.
+<hexboard size="4x8"
+  coords="hide"
+  edges="bottom"
+  visible="-a4 b4 c4 h3 h4"
+  contents="R a1 a3 b3 c3 d3 B 1:f4 R 2:e2 B 3:d4 R 4:f3 B 5:e4 R 6:g3"
+  />
+This finishes the proof of the lemma. □
+Having reduced the distance ''n'' to finitely many cases, we would now like to reduce the parameter ''k'' to finitely many cases as well. The following lemma does this.
+'''Lemma 21.''' Suppose P is a candidate for a 2-4 terraced ladder escape. Assume T(0)+P, T(1)+P, and T(2)+P are virtual connections (i.e., they guarantee a connection of the top ladder stone to the edge, with Blue to move). Then T(''k'')+P is a virtual connection for all ''k'' ≥ 0.
+'''Proof.''' The first step in the proof is to show that the following two interior patterns are equivalent. By "interior pattern", we mean that the bottom row of red stones does not have to be a board edge.
+B(1): <hexboard size="4x3"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 d4"
+  contents="B c1 R a2 a4--c4 E x:c2 y:c3 z:a3 -:(d1--d3)"
+  />
+B(2): <hexboard size="4x4"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 e4"
+  contents="B c1--d1 R a2 a4--d4 E x:d2 y:d3 z:a3 -:(e1--e3)"
+  />
+If Red plays first in the region, a red move at x [[captured cell|captures]] the entire region, so x is the only move that Red needs to consider, and its outcome is the same in B(1) and B(2).
+If Blue moves first in the region, all of the interior moves (i.e., in unmarked cells) are [[Dominated_cell#Star_decomposition_domination|star-decomposition dominated]] by x. Therefore, Blue only needs to consider the moves x, y, and z. One can show that each of these three moves (x, y, and z) in region B(2) is equivalent to the corresponding move in region B(1). For example, after Blue moves at x, z dominates all of the interior moves and whoever plays there [[captured cell|captures]] the interior, regardless of whether the region is B(1) or B(2).
+A consequence of the fact that regions B(1) and B(2) are equivalent is that all "longer" versions of these regions are also equivalent to B(1), B(2), and each other, i.e.,
+B(3): <hexboard size="4x5"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 f4"
+  contents="B c1--e1 R a2 a4--e4 E -:(f1--f3)"
+  />
+B(4): <hexboard size="4x6"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 g4"
+  contents="B c1--f1 R a2 a4--f4 E -:(g1--g3)"
+  />
+and so on. This is easily proved by induction, because each longer region is obtained from the previous one by replacing a subregion of the form B(1) by B(2), which we already showed to be equivalent.
+Next, consider this pattern:
+B(0): <hexboard size="4x2"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 c4"
+  contents="R a2 a4--b4 E x:b2 y:b3 z:a3 -:(c1--c3)"
+  />
+We claim that B(1) is at least as good as B(0) for Red, in the sense that anything Red can achieve with B(0), Red can also achieve with B(1). (In fact, B(1) is strictly better for Red than B(0), but that fact is not required for this proof). If Red moves first in the region B(0), the move at x again captures the whole region, and therefore achieves everything Red might hope to achieve in the region. In this case, B(0) and B(1) are equivalent. If Blue moves first, the situation is slightly more complicated. We must show that B(0) is at least as good for Blue as B(1). If Blue plays at x in B(1), then Blue has the corresponding option to move at x in B(0), which works for the same reason as in the proof of the equivalence of B(1) and B(2) above. If Blue plays at z in B(1), Red can respond by pushing the ladder, which creates a position that is literally B(0). If Blue plays at y in B(1), Red can respond at x, and a case distinction shows that no matter how the remaining 3 cells are filled, filling them in the same way in B(0) gives an equivalent position.
+Finally, let C(0), C(1), C(2), ... be the same patterns as B(0), B(1), B(2), ..., except with the blue stones removed from the carrier. I.e.:
+C(0): <hexboard size="4x2"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 c4"
+  contents="R a2 a4--b4 E -:(c1--c3)"
+  />
+C(1): <hexboard size="4x3"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 d4 c1"
+  contents="R a2 a4--c4 E -:(d1--d3)"
+  />
+C(2): <hexboard size="4x4"
+  coords="none"
+  edges="none"
+  visible="-a1 b1 e4 c1--d1"
+  contents="R a2 a4--d4 E -:(e1--e3)"
+  />
+etc. Note that for all ''k'' ≥ 0, C(''k'') is at least as good for Red as B(''k''). Because if the neighboring cells we removed from the templates are in fact occupied by Blue, then C(''k'') is the same as B(''k''); otherwise, if they are empty or Red, it can only help Red.
+In particular, since each C(''k'') is at least as good for Red as B(''k''), and each B(''k'') is at least as good as B(0) = C(0), it follows that if Red wins any position containing C(0), then Red also wins the corresponding position containing C(''k'').
+The final step in the proof is now easy. Simply observe that each T(''k''+2) is obtained from T(2) by replacing a subpattern of the form C(0) by C(''k''). Therefore, in any context P where T(2)+P is winning for Red, T(''k''+2)+P is also winning for Red. Combining this with the remaining two base cases T(0)+P and T(1)+P, we get the lemma. □
+By combining the previous two lemmas, we obtain a sufficient condition for the validity of terraced ladder escapes.
+'''Theorem 22.''' Consider a candidate P for a 2-4 terraced ladder escape. Assume T(''k'')+''n''+P is a virtual connection for ''n''=1,2,3 and ''k''=1,2,3 (nine possibilities). Moreover, assume that ∗+P escapes 2nd, 3rd, and 4th row ladders (or more precisely, ∗+P escapes 4th row ladders, ↑+∗+P (which has 3 cells marked "+") escapes 3rd row ladders, and ↑+↑+∗+P escapes 2nd row ladders). Then P is a valid 2-4 terraced ladder escape.
+'''Proof.''' By Lemma 21, T(''k'')+''n''+P is a virtual connection for ''n''=1,2,3 and all ''k'' ≥ 0. Therefore, the hypothesis of Lemma 20 is satisfied, and thus P is valid. □
+=== Non-examples ===
+Since terraced ladders are weaker than 4th row ladders, any terraced ladder escape is also a 4th row ladder escape. The question then becomes: which 4th row ladder escapes are ''not'' terraced ladder escapes? Most, but not all, of the examples of 4th row ladder escapes given above also escape terraced ladders.
+The following patterns escape 4th row ladders but do not escape terraced ladders:
+<hexboard size="5x6"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 e1 f1 f2"
+   contents="E *:a1 E +:a2 E +:a3 E +:a4 E +:a5 R d2 E *:e1 E *:f1 E *:f2"
+   />
+<hexboard size="6x5"
+   coords="hide"
+   edges="bottom"
+   visible="-a1 a2 b1 c1"
+   contents="E *:a1 E *:a2 E +:a3 E +:a4 E +:a5 E +:a6 E *:b1 E *:c1 R c2 R d1"
+   />
-To push this theory to the 5th and 6th row, one first needs to come up with a rigorous definition of a ladder (the issue being how much space one needs under the ladder stone to stop blue from cutting off the ladder immediately). Once this is done one needs to come up with a finite list of templates analogous to, say, A to D in the 4th row argument above. This may not be as easy as it looks. For example B above is A plus one extra row, and when I tried to prove the result initially I only had A, C and D, and I could not get the argument to work; the extra space was essential. More issues of this nature surely await anyone who tries to extend these results to 5th row ladders or higher. Because I am not sure that 5th row ladder escapes really ever come up in practice, I am not particularly motivated to pursue this.
+[[category: Theory]]
+[[category: Ladder]]

Difference between revisions of "Theory of ladder escapes"

Latest revision as of 18:14, 19 October 2024

Contents

Algebraic notation

Open patterns

Addition

The shift operation

The reduce operation

Second row ladders

Definition of ladder

Definition of ladder escape

Characterization of ladder escapes

Examples

Third row ladders

Definition of ladder

Definition of ladder escape

Characterization of ladder escapes

Examples

Fourth row ladders

Definition of ladder

Definition of ladder escape

Characterization of ladder escapes

Examples

Fifth row ladders

Definition of ladder

Definition of ladder escape

Characterization of ladder escapes

Examples

Sixth row ladders and up

Second-to-fourth row switchbacks

Definition of switchback

Characterization of switchbacks

Examples

Third-to-fifth row switchbacks

Definition of switchback

Characterization of switchbacks

Examples

Second and fourth row parallel ladders

Definition of ladder

Definition of ladder escape

Characterization of ladder escapes

Examples

Third and fifth row parallel ladders

Definition of ladder

Definition of ladder escape

Characterization of ladder escapes

Examples

Second and fourth row terraced ladders

Definition of ladder

Definition of ladder escape

Characterization of ladder escapes

Non-examples

Navigation menu

Search