# Transfinite Nim: uncomputable games and games whose winner depends on the Continuum Hypothesis

In the game of Nim, you start with piles of various (whole number) heights. Each step, a player chooses one pile and shrinks it by some non-zero amount. Once a pile’s height has been shrunk to zero, it can no longer be selected by a player for shrinking. The winner of the game is the one that takes the last pile to zero.

Here’s a sample game of Nim:

Starting state
3, 2
After Frank’s move
2, 2
After Marie’s move
2, 1
After Frank’s move
0, 1
After Marie’s move
0, 0

Marie takes the last pile to zero, so she is the winner. Frank’s second-to last move was a big mistake; by reducing the first pile from 2 to 0, he left the only remaining pile free to be taken by Marie. In a game of Nim, you should never leave only one pile remaining at the end of your turn. If Frank had instead shrunk the first pile from 2 to 1, then the state of the piles would be (1, 1). Marie would be forced to shrink one of the two piles to zero, leaving Frank to take the final pile and win.

The strategy of Nim with two piles is extremely simple: in your turn you should always even out the two piles if possible. This is only possible if the heights are different at the start of your turn. See if you can figure out why this strategy guarantees a win!

Transfinite Nim is a version of Nim where the piles are allowed to take infinite ordinal values. So for instance, a game might have the following starting position:

Starting state
ω2 + ω, ω1 + ε0

If Marie is moving first, then can she guarantee a win? What move should she make?

It turns out that the strategy for two-pile Transfinite Nim is exactly the same as for two-pile Finite Nim. Marie has a guaranteed win, because the two piles are different values. Each move she’ll just even the piles out. So for her first move, she should do the following:

Starting state
ω2 + ω, ω1 + ε0
After Marie’s move
ω2 + ω, ω2 + ω

No matter what Frank does next, Marie can just “copy” that move on the other pile, guaranteeing that Marie always has a move as long as Frank does. This proves that Marie must have the last move, and therefore win.

One important feature of Transfinite Nim is that even though we’re dealing with infinitely large piles, every game can only last finitely long. In other words, Frank has no strategy for delaying his loss infinitely long, and thus forcing a sort of “stalemate by exhaustion.” This is because the ordinals are well-ordered, and any decreasing sequence of well-ordered items must terminate. (Why? Just consider the definition of a well-ordered set: every subset has a least element. If the game were to continue infinitely long, each step decreasing the state but never terminating, then the sequence of states would be a subset of the ordinals which has no least element!)

Although the strategy of Transfinite Nim is in one sense no more interesting than Finite Nim, the game does have some interesting features that it inherits from the ordinals. For instance, there are sets of ordinal numbers such that the ordering between them is uncomputable. For such sets, the ability to compute the winning strategy is called into question.

For instance, the set of all countable ordinals is uncomputable. The quick proof is that there are uncountably many countable ordinals – otherwise in ZFC the set of countable ordinals would itself be a countable ordinal and would thus contain itself – and any Turing machine can only compare countably many things. However, there are also uncomputable ordinals that are countable! If α is a countable ordinal, then we can find some bijection (not necessarily order-preserving) between α and ω, meaning that we can meaningfully ask if a Turing machine can compare any two of α’s elements (each represented by some natural number). And for an uncomputable countable ordinal, we know that no Turing machine can successfully compute its order type.

The smallest uncomputable ordinal (which, in ZFC, is exactly the set of all computable ordinals) is called the Church Kleene ordinal and written ω1CK. Imagine the starting state of the game is two different ordinals that are both larger than ω1CK. If you’re moving first, then you have to determine which of the two ordinals is larger, in order to even them out. But this is not in general possible! So even if you go first and the two piles are different sizes, you might not be able to guarantee a win.

Suppose Marie is allowed uncomputable strategies, and Frank is only allowed computable strategies. Suppose further that the starting state involves two countable ordinals A and B, both larger than the Church-Kleene, and that the ordinals are expressed in some standard notation (so that you can’t write the same ordinal two different ways). There are a few cases.

Case 1: A = B, Marie goes first.
Marie decreases one of the two ordinals. Despite not being able to compute the order on the ordinals, Frank can just mimic her move. This will continue until Frank wins.

Case 2: A = B, Frank goes first.
Frank decreases one of the two ordinals, and Marie mimics. Marie eventually wins.

Case 3: A ≠ B, Marie goes first.
Marie can tell which of the ordinals is larger, and decreases that one to even out the two piles. Marie wins.

Case 4: A ≠ B, Frank goes first.
Frank can’t tell which of the ordinals is larger and can’t try to even them out, as doing so might result in an invalid move (trying to increase the smaller pile to the height of the larger one). So Frank does some random move, after which Marie is able to even out the two piles. Marie wins.

There’s a subtlety in Case 4, which is that Frank could gamble by guessing that B is the bigger ordinal and then decreasing it to A. If he has no other information, then half the time he’ll end up successfully evening them out, in which case he continues to win the game. But the other half of the time he’ll have made an invalid move. If we assume that players cannot run strategies that have some chance of choosing invalid moves (for instance, if each player has to be able to prove that their move is valid in advance), then Frank’s gamble would not be allowed and he would go on to lose.

Finally, here’s a starting state for a game of Transfinite Nim:

ω1, ℶ1

ω1 is the first uncountable ordinal, and ℶ1 is the first ordinal with continuum cardinality. Frank goes first. Does he have a winning strategy?

The answer to this question depends on whether ω1 = ℶ1, or in other words the Continuum Hypothesis! If the two are equal, then Frank can’t win, because he’s starting with two even piles. And if ω1 < ℶ1, then Marie can’t win, because Frank can decrease the ℶ1 pile to ω1.

If we suppose that the players must be able to prove a move’s validity in ZFC before playing that move, then the first player couldn’t decrease the ℶ1 pile to ω1. The first player still has to do something, and whatever he does will change the state to two ordinals that are comparable by ZFC. What about larger starting ordinals whose size comparison is independent of ZFC, like ω15 and ℶ15? If the new state after the first player’s move move also involves two ordinals whose size comparison is independent of ZFC, then the second player will also be unable to even them out. This continues until one of them eventually decreases a pile to an ordinal whose size is comparable by ZFC to the other pile. So the winner will depend on who knows more pairs of ordinals less than the starting values with values that ZFC can’t compare. In fact, each player wants to force the other player to make the values ZFC-comparable, so they’ll be able to even the piles out on their turn.

What if our players are allowed to use different proof systems from each other? Then adjudication of whether a move is valid requires that we fix some meta-theoretic proof system as our judge. For instance, suppose our meta-theory is ZFC + V=L (in which case ω1 does equal ℶ1). In this case, if a player is using a theory from which they can prove that ω1 < ℶ1, they might end up making a move that we judge as invalid, even though in their view it’s perfectly valid. Presumably then each player has to reason within their own theory about what is valid according to the judge’s meta-theory. But perhaps these judgements will be fallible! If so, then the victor may end up depending on who has a better theory of the judge’s meta-theory!