# 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!

# A Coloring Problem Equivalent to the Continuum Hypothesis

Summary
Is there an infinity in between ℵ0 and 20? Is there a way to color the plane with countably many colors so that there are no monochromatic triangles lined up with the x and y axes? If these questions seem unrelated to you, then we had the same initial reaction. But it turns out that an intermediate infinity exists if and only if no such coloring exists.

CH and Coloring

Imagine taking the plane ℝ2 and coloring it entirely. You are allowed infinitely many colors, but only a countable infinity (so no using the whole continuous spectrum). Some example colorings:

The colorings don’t have to be contiguous on the plane like in the ones you just saw. They can have individual points of one color such that neighborhoods of arbitrarily small radius around them are colored differently. So we can get wacky colorings like:

Think about right triangles on the plane that line up with the x and y axes. For instance:

For each of these triangles, of the three sides one must be parallel to the x-axis and another to the y-axis. Now, for a given coloring, we can ask if there exist any triangles whose corners are all the same color. Importantly, we’re only interested in the three corner points, not the entire side length. For the earlier colorings we looked at, we can find such triangles fairly easily:

Even for our wacky coloring, it shouldn’t be too hard to find three points of the same color that meet the criterion.

Now here’s a question for you to ponder: is there any way to color the plane so that no such monochromatic right triangles exist?

I encourage you to pause here and try your best to come up with such a coloring. Use this as a chance to develop some intuitions on whether you think that a coloring like this should exist or not. It’s important to keep in mind that the amount of possible right triangles on the plane is 20, while the number of colors you have available to you is just ℵ0.

(…)

(pause for thought)

(…)

Now, I shall prove to you that the existence of such a coloring is equivalent to the Continuum Hypothesis! The rest of this post will be more involved, and even though I’ve tried to make it less scary with pictures aplenty, it will most likely take a slow and attentive read-through to grasp. But the proof is really cool, so it’s worth it. Oh and also, I have assumed some things as background knowledge for the sake of space. In particular, understanding the proof requires that you understand what ordinals are and why the set of all countable ordinals is the first uncountable ordinal.

If the Continuum Hypothesis is true, then a coloring with no monochromatic right triangles exists.

The proof outline: Consider the first uncountable ordinal, ω1, which is the set of all countable ordinals. We construct a coloring on ω1 × ω1 that satisfies the no-monochromatic-right-triangles property. If the Continuum Hypothesis is true, then |ω1| = |ℝ|, so we form a bijection from ω1 to ℝ. Finally, we use this bijection to map our coloring on ω1 × ω1 to a coloring on ℝ × ℝ, and show that coloring also satisfies the same property.

Alright, so consider ω1 × ω1. If we were to visualize it, it would look something this:

This “plane” is discrete, so in a certain sense it much more closely resembles ℕ2 than ℝ2. The set of colors we’ll be using will be labeled with integers. We split the colors into two categories, the negatives N and the positives P.

N = {-1, -2, -3, …}
P = {0, 1, 2, 3,…}

For each α in ω1, we construct two bijections f: α → N and g: α → P. These bijections exist because every element of ω1 is countable.

We use f and g to construct our coloring. For each α in ω1, we assign the color f(β) to (α, β) for each β < α, and we assign the color g(β) to (β, α), for each β ≤ α.

Since f and g are both bijections, no colors are repeated along the path from (α, 0) to (α, α) to (0, α). And we can easily show that no monochromatic right triangles exist under this coloring!

If a triangle crosses y = x, then it has at least one point colored from N and another from P. N and P are disjoint, so the triangle cannot be monochromatic.

And if a triangle doesn’t cross y = x, then it has two points lined up so as to have different colors:

So we have no monochromatic triangles! Now, assuming the Continuum Hypothesis, |ω1| = |ℝ|, so we can form a bijection M from ω1 to ℝ. We use M to map our original coloring to a new coloring on ℝ × ℝ. If (a, b) ∈ ω1 × ω1 is colored C, then we also color (M(a), M(b)) ∈ ℝ × ℝ. Under this map, non-monochromatic triangles stay non-monochromatic. So we’ve constructed a coloring on ℝ × ℝ with no monochromatic triangles!

If a coloring with no monochromatic right triangles exists, then the Continuum Hypothesis is true.

The proof outline here is to show that any infinite subset A of ℝ either has cardinality ℵ0 or 20. This shows that there are no intermediate cardinalities between these two.

Consider any infinite subset A ⊆ ℝ. For any a, define Ya to be the y-coordinates of points along the vertical line through (a, 0) that have no color repeats on that line. So Ya = {y ∈ ℝ | the color of (a, y) is different from (a, y’) for all y’ ≠ y}.

Notice that Ya must be countable, because all its elements have different colors and there are only countably many different colors to have. So |Ya| = ℵ0.

Define YA to be the union of Ya, for each a in A. Since each Ya is countable and A is at least ℵ0, |YA| ≤ |A|⋅ℵ0 = |A|. There are two cases: either YA = ℝ, or YA ≠ ℝ.

Case 1: YA = ℝ. In this case |ℝ| = |YA| ≤ |A|, so |A| ≥ |ℝ|. Since A is a subset of ℝ, this implies that |A| = |ℝ|.

Case 2: YA ≠ ℝ. In this case, there’s some real number y* in ℝ – YA. By definition y* is outside YA, so for each vertical line {x} × ℝ there is at least one point with the same color as (x, y*) besides itself.

Each (x, y*) with x ∈ A must have a different color. Proof: Suppose not. Then we have two elements of A, x1 and x2, such that the color of (x1, y*) is the same as the color of (x2, y*). By the last paragraph’s result, we can pick a third point (x1, y’) that has the same color as (x1, y*). These three points form a monochromatic right triangle, but we’ve assumed that’s impossible! Contradiction

So each point in A × {y} must have a different color. Thus, the cardinality of A × {y} must be countable. A is an infinite set, so |A| = ℵ0.

So in case one |A| = |ℝ| = 20, and in case two |A| = ℵ0. And this is true for every infinite subset A of ℝ. So every subset of ℝ is either finite, countable, or has the same cardinality as ℝ. And this is the continuum hypothesis!