G13GAM -- Game Theory

Impartial Hackenbush

[This is a copy of http://www.maths.nottingham.ac.uk/personal/anw/G13GAM/hack.html]

Impartial Hackenbush can be completely solved by the three results:

Every non-empty picture can be evaluated using these principles: the fusion principle allows us to get rid of all loops, leaving a picture which is a collection of [mathematical this time, but hence biological!] trees which can be evaluated inductively from the tree principle. Of course, with experience, much of this process can be short-cut.

The Colon Principle

In the figure, let us suppose that H and I, viewed as pictures in their own right, have equal value, and that they are joined to the rest of their components, G, only at the `articulation point' marked by the red dot. [The colours in this and other figures here are purely decorative and to draw attention to parts of the picture; they are not meant to indicate that parts `belong' to one or other player.] Then the two components, which we can write as G:H and G:I, also have the same value. [More generally, if H>=I, then G:H>=G:I. Impartial pictures are always either zero or fuzzy (first-player wins), so this generalisation is unimportant here.]
Proof: Play the difference game, G:H-G:I. [For impartial games, equal to their own negatives, the difference game is the same as the disjunctive sum of the games.] Since H-I = 0, to every move in either H or -I there is a winning reply in H-I; this same winning reply can be played in response to a move in the colon'ed version, and then we can appeal to induction, as the resulting picture is a simpler one to which the colon principle still applies. Equally a move in G has a symmetric counter in -G and vice versa; this move and counter either delete the articulation points and all that sail on them in both components [giving a zero game] or in neither, in which case we can again appeal to induction and the colon principle.
Note that this works perfectly well even in general Hackenbush, but we need it here only in the impartial version. In the impartial version, the value of H will perforce be a nimber, so the component I might just as well be a `snake', a simple chain of n edges, n>=0, value *n, of the appropriate length. In the particular case where G is just a single edge, this shows that G:H is equivalent to a snake of length n+1, which is the tree principle.

The Fusion Principle

Suppose we call a picture good if the FP applies to it, and bad if it doesn't -- that is, if the picture includes a loop whose fusion would change its value. Suppose bad pictures exist. Then there must be simplest possible bad pictures, meaning with as few edges as possible, and among those with the same number of edges one with the fewest possible nodes [counting the ground as one node]. Choose such a simplest bad picture, G. Then: We conclude the proof by showing that such a G is in fact good, contrary to supposition. Note that any move in G either makes a simpler bridge, which is good by supposition [and can be evaluated by the fusion principle], or [by removing a span of the bridge] makes two trees, whose value we can work out. So this is now a technical problem of evaluating a fairly specific picture, which you can take on trust, or see the next section.

Bridges are good!

If we fuse a bridge, then we turn each span of the bridge into a blade of grass, and each snake becomes attached instead to the ground. The blades of grass cancel in pairs. so the FP tells us that the value of a bridge is the value of its snakes, together with a blade of grass if the number of spans is odd. So we need to show that a picture typified by this one has value zero, i.e. is a first-player loss. Clearly any move in any snake loses; the other player can make the same move in the corresponding snake, to give a simpler bridge, which must be good [or we can use induction].

What about moves in the bridge, typified by the greyed-out edge? These too lose. Such a move creates two trees. Recall that to evaluate a tree, we have to Nim-add branches, add one to go down, Nim-add, add one, etc. As Nim addition is adding without carry, the least-significant [binary] digit works out the same whether we Nim-add or ordinary-add; in other words, if we replace all the Nim-adds by ordinary-adds, the parity of the answer will be the same. From this, it is easily seen that the parity of the value of the tree is the same as the parity of its number of edges. Now, whether we started out with an odd or an even bridge, that bridge plus its fused version has an even number of edges [each snake occurs twice, and there is a blade of grass only if the number of spans is odd]. So when we chop a span, we have an odd number of edges; we have just demolished the last loop, so the whole picture is the Nim-sum of a collection of trees, and so is an odd nimber, so cannot be zero. In other words, any move in the bridge creates a picture which is a win for the other player.

That completes the result in the case where the bridge is even. When the bridge is odd, as in the picture shown, there is one final move, to mow the grass. The resulting picture has an odd number of edges, but it isn't a tree. Any chop of a bridge span will create trees with, in total, an even number of edges, but they could possibly all be *2 or *4 or whatever. Can we move in a snake? Well, if any snake on the bridge is odd, then chop off its head. The resulting picture is good [as it is simpler than the one we started with], so its value is lots of snakes that cancel out in pairs, an odd bridge that fuses to *1, the original odd snake from the ground worth *(2n+1) and the chopped snake from the bridge worth *(2n), total zero, so the chop was a good reply. So, we are left with the case where they are all even.

We finally have to show that in a bridge with an odd number of spans and a collection of even snakes, plus copies of all the snakes, there is a move to zero. Call the centre span `white', and then label spans alternately black and white out towards the ends of the bridge. It suffices to show that we can win by chopping some white span. Suppose we chop such a span. What is the resulting value of the two trees? Well, each has Nim-value

2a + 1 @ 2b + 1 @ 2c + 1 @ 2d + 1 @ ...
[where `@' means Nim addition and lots of parentheses have been omitted, but the expression should be worked out strictly from left to right], where 2a, 2b, ... are the snake heights working out from the chopped edge. But now if you look at the way the odds and evens go, the first, third, fifth, ... `+ 1' have the same effect as `@ 1', which means that they commute with the next `@ 2b', and the sum is the same as the nimber
2a @ 2b + 2 @ 2c @ 2d + 2 @ ...
Now, everything in this is even, so the result is twice the result of
a @ b + 1 @ c @ d + 1 @ ...
But this is what you get if you halve each snake, then close up each black span to a point, Nim-adding the snakes at each end. [Each of the trees has a black span at its end, as we chopped a white span.] The resulting picture is good, and clearly has Nim-value 1, [or game value *1 = *]. [It has lots of snakes that can be paired off, and an odd-span bridge, as there are an odd number of white spans.] So, some move in the reduced bridge wins, i.e. chops it to zero, and therefore the corresponding move in the original bridge also chops to [twice] zero. That concludes the proof.
The very astute will have noted a flaw: The win in the reduced bridge might not be by chopping a bridge span, but by chopping a snake. The fusion principle, as applied to the simpler picture, shows that you can in fact chop a bridge span instead, but finding which one is decidedly non-trivial. The way to proceed is a generalisation of the above `black-white' idea; for `how to do it', see Winning Ways, p189.
None of the above is examinable!

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