## A Short Proof of the Nash-Williams’ Partition Theorem

Notations

1. $$\mathbb{N}$$ – the set of natural numbers;
2. $$\binom{M}{k}$$ – the family of all subsets of $$M$$ of size $$k$$;
3. $$\binom{M}{<\omega}$$ – the family of all finite subsets of $$M$$;
4. $$\binom{M}{\omega}$$ – the family of all infinite subsets of $$M$$;

The infinite Ramsey theorem, in its simplest form, states that for every partition $$\binom{\mathbb{N}}{k} = \mathcal{F}_1 \sqcup \dots \sqcup \mathcal{F}_r$$, there exists an infinite set $$M\subset \mathbb{N}$$ such that $$\binom{M}{k}\subset \mathcal{F}_i$$ for some $$i\in [r]$$. The Nash-Williams‘ partition theorem can be seen as a strengthening of the infinite Ramsey theorem, which considers a partition of a subset of $$\binom{\mathbb{N}}{<\omega}$$.

Notations

1. $$\mathcal{F}\restriction M$$ – $$\mathcal{F}\cap 2^M$$, that is, the set $$\{s\in\mathcal{F} : s\subset M\}$$.
2. $$s \sqsubset t$$, where $$s,t$$ are subsets of $$\mathbb{N}$$ – $$s$$ is an initial segment of $$t$$, that is $$s = \{n\in t : n \le \max s\}$$.

Definition Let set $$\mathcal{F} \subset \binom{\mathbb{N}}{<\omega}$$.

1. $$\mathcal{F}$$ is Ramsey if for every partition $$\mathcal{F}=\mathcal{F}_1\sqcup \dots\sqcup\mathcal{F}_r$$ and every $$M\in\binom{\mathbb{N}}{\omega}$$, there is $$N\in\binom{M}{\omega}$$ such that $$\mathcal{F}_i\restriction N = \emptyset$$ for all but at most one $$i\in[r]$$.
2. $$\mathcal{F}$$ is a Nash-Williams family if for all $$s, t\in\mathcal{F}, s\sqsubset t \implies s = t$$.

Theorem [NASH-WILLIAMS 1965] Every Nash-Williams family is Ramsey.

The proof presented here is based on the proof given by Prof. James Cummings in his Infinite Ramsey Theory class. The purpose of this rewrite is to have a proof that resembles the one of the infinite Ramsey theorem.

Notation Let $$s\in\binom{\mathbb{N}}{<\omega}$$ and $$M\in\binom{\mathbb{N}}{\omega}$$. Denote $$[s, M] = \left\{t \in \binom{\mathbb{N}}{<\omega} : t \sqsubset s \text{ or } (s \sqsubset t \text{ and } t\setminus s \subset M)\right\}.$$

Definition Fix $$\mathcal{F}\subset \binom{\mathbb{N}}{<\omega}$$ and $$s\in \binom{\mathbb{N}}{<\omega}$$.

1. $$M$$ accepts $$s$$ if $$[s, M]\cap \mathcal{F}\neq \emptyset$$ and $$M$$ rejects $$s$$ otherwise;
2. $$M$$ strongly accepts $$s$$ if every infinite subset of $$M$$ accepts $$s$$;
3. $$M$$ decides $$s$$ if $$M$$ either rejects $$s$$ or strongly accepts it.

We list some properties that encapsulates the combinatorial characteristics of the definitions above.

Properties

1. If $$M$$ decides (or strongly accepts, or rejects) $$s$$ and $$N\subset M$$, then $$N$$ decides (respectively strongly accepts, rejects) $$s$$ as well.
2. For every $$M\in\binom{\mathbb{N}}{\omega}$$ and $$s\in\binom{\mathbb{N}}{<\omega}$$, there is $$N_1\in\binom{M}{\omega}$$ deciding $$s$$. Consequently, there is $$N_2\in\binom{M}{\omega}$$ deciding every subset of $$s$$.

Proof of Theorem Enough to show that if $$\mathcal{F} = \mathcal{F}_1\sqcup \mathcal{F}_2$$, then for every $$M\in\binom{\mathbb{N}}{\omega}$$, there is infinite $$N\in \binom{M}{\omega}$$ such that $$F_i \restriction N = \emptyset$$ for some $$i\in[2]$$.

We are going to use $$\mathcal{F}_1$$ instead of $$\mathcal{F}$$ in the definitions of “accept”, “reject”, “strongly accept” and “decide”. Find $$N\in \binom{M}{\omega}$$ that decides $$\emptyset$$. If $$N$$ rejects $$\emptyset$$, by definition $$\mathcal{F}_1\restriction N = [\emptyset, N]\cap \mathcal{F}_1 = \emptyset$$. Otherwise $$N$$ strongly accepts $$\emptyset$$.

Inductively, we build a decreasing sequence of infinite sets $$N \supset N_1 \supset N_2\supset \dots$$, an increasing sequence of natural numbers $$n_1, n_2, \dots$$, and maintain that $$n_i\in N_i, n_i < \min N_{i+1}$$ and that $$N_i$$ strongly accepts every $$s\subset \{n_j : j < i\}$$. Initially, we take $$N_1 = N$$ as $$N$$ strongly accepts $$\emptyset$$.

Suppose $$N_1 \supset \dots \supset N_i$$ and $$n_1 < \dots < n_{i-1}$$ have been constructed. In the following lemma, when taking $$M = N_i$$ and $$s = \{n_j : j < i\}$$, it spits out $$m$$ and $$N$$, which are exactly what we need for $$n_i$$ and $$N_{i+1}$$ to finish the inductive step.

Lemma Suppose $$M\in\binom{\mathbb{N}}{\omega}$$, $$s\in\binom{\mathbb{N}}{<\omega}$$ and $$\max s < \min M$$. If $$M$$ strongly accepts every subset of $$s$$, then there are $$m \in M$$ and $$N \in \binom{M}{\omega}$$ such that $$n < \min N$$ and $$N$$ strongly accepts every subset of $$s\cup \{n\}$$

Proof of lemma We can build $$M = M_0 \supset M_1\supset M_2 \supset \dots$$ such that for every $$i$$, $$m_i := \min M_i < \min M_{i+1}$$ and $$M_{i+1}$$ decides every subset of $$s\cup \{m_i\}$$. It might happen that $$M_{i+1}$$ rejects a subset of $$s\cup \{m_i\}$$. However, we claim that this cannot happen for infinitely many times.

Otherwise, by the pigeonhole principle, there is $$t\subset s$$ such that $$I = \{i : M_{i+1} \text{ rejects }t\cup\{m_{i}\}\}$$ is infinite. Let $$M’ = \{m_i : i\in I\}$$. Note that $$[t, M’] \subset \cup_i [t\cup\{m_i\}, M_{i+1}]$$, and so $$[t,M’]\cap \mathcal{F}_1\subset \cup_i \left([t\cup\{m_i\}, M_{i+1}]\cap\mathcal{F}_1\right) = \emptyset$$. Hence $$M’\subset M$$ rejects $$t\subset s$$, which is a contradiction.

Now we pick one $$i$$ such that $$M_{i+1}$$ strongly accepts every subset of $$s\cup\{m_i\}$$, and it is easy to check that $$m = m_i$$ and $$N = M_{i+1}$$ suffice. QED for lemma.

Finally, we take $$N_\infty = \{n_1, n_2, \dots\}$$. For any $$s\in\binom{N_\infty}{<\omega}$$, there is $$i$$ such that $$s\subset \{n_1, \dots, n_{i-1}\}$$. Note that $$N_i$$ strongly accepts $$s$$ and $$N_\infty\subset N_i$$. Therefore $$N_\infty$$ (strongly) accepts $$s$$, that is $$[s, N_\infty]\cap \mathcal{F}_1 \neq \emptyset$$, and say $$t\in [s, N_\infty]\cap \mathcal{F}_1$$. Because $$t\in\mathcal{F}_1$$ and $$\mathcal{F} = \mathcal{F}_1 \sqcup \mathcal{F}_2$$ is a Nash-Williams family, $$s\notin \mathcal{F}_2$$. QED.

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## An Upper Bound on Stirling Number of the Second Kind

We shall show an upper bound on the Stirling number of the second kind, a byproduct of a homework exercise of Probabilistic Combinatorics offered by Prof. Tom Bohman.

Definition A Stirling number of the second kind (or Stirling partition number) is the number of ways to partition a set of $$n$$ objects into $$k$$ non-empty subsets and is denoted by $$S(n,k)$$.

Proposition For all $$n, k$$, we have $$S(n,k) \leq \frac{k^n}{k!}\left(1-(1-1/m)^k\right)^m.$$

Proof Consider a random bipartite graph with partite sets $$U:=[n], V:=[k]$$. For each vertex $$u\in U$$, it (independently) connects to exactly one of the vertices in $$V$$ uniformly at random. Suppose $$X$$ is the set of non-isolated vertices in $$V$$. It is easy to see that $$\operatorname{Pr}\left(X=V\right) = \frac{\text{number of surjections from }U\text{ to }V}{k^n} = \frac{k!S(n,k)}{k^n}.$$

On the other hand,  we claim that for any $$\emptyset \neq A \subset [k]$$ and $$i \in [k]\setminus A$$, $$\operatorname{Pr}\left(i\in X \mid A\subset X\right) \leq \operatorname{Pr}\left(i\in X\right).$$ Note that the claim is equivalent to $$\operatorname{Pr}\left(A\subset X \mid i\notin X\right) \geq \operatorname{Pr}\left(A\subset X\right).$$ Consider the same random bipartite graph with $$V$$ replaced by $$V’:=[k]\setminus \{i\}$$ and let $$X’$$ be the set of non-isolated vertices in $$V’$$. The claim is justified since $$\operatorname{Pr}\left(A\subset X\mid i\notin X\right) = \operatorname{Pr}\left(A\subset X’\right) \geq \operatorname{Pr}\left(A\subset X\right).$$

Set $$A:=[i-1]$$ in above for $$i = 2, \ldots, k$$. Using the multiplication rule with telescoping the conditional probability, we obtain $$\begin{eqnarray}\operatorname{Pr}\left(X=V\right) &=& \operatorname{Pr}\left(1\in X\right)\operatorname{Pr}\left(2\in X \mid [1]\subset X\right)\ldots \operatorname{Pr}\left(k\in X\mid [k-1]\subset X\right)\\ & \leq & \operatorname{Pr}\left(1\in X\right)\operatorname{Pr}\left(2\in X\right)\ldots\operatorname{Pr}\left(k\in X\right) \\ & = & \left(1-(1-1/m)^k\right)^m.\end{eqnarray}$$ QED.

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## Fun with Hex (Cont.)

In the previous post Fun with Hex, I asserted without proof that the Hex Game will never result a draw. In Jiri Matousek’s An Invitation to Discrete Mathematics, I found an elegant proof for the assertion, which requires only a little bit of elementary graph theory. The idea of the proof can also prove Sperner’s Lemma. Moreover, I will present later how to extrapolate this idea to discover something new. The only bit of elementary graph theory we are going to use is the Handshaking lemma.

Handshaking Lemma: Every finite undirected graph has an even number of vertices with odd degree.

To prove the lemma, simply observe that the sum of the degrees of all vertices is equal to twice the number of the edges, which should be even.

Now let us review the Hex Game. Recall two versions of the Hex Game.

The original version of the game is to color the hexagons of the game board (illustrated in black) and each player tries to link the opposite sides of the board with a connected series of hexagons with the same color.

Alternatively, one can play with the Voronoi dual of the hexagon game board (illustrated in red). This time, two players color the vertices and compete to link the opposite sides with a path of vertices of their own color. For instance, player one would try to form a red path from the northwest side to the southeast side, while player two would strive to build a blue one from the northeast side to the southwest side.

We can tilt and rescale the dual graph without changing the game as follows.

In this new graph, the first player wants to build a red path from the west to the east and the second player wants to build a blue path from the north to the south. Furthermore, for purpose of the proof, we add four auxiliary vertices on each side and color them according to which player it belongs to.

As are shown in the above graph, the vertices on the west and the east are red and the ones on the north and the south are blue. Our goal is to prove that no matter how you color the remaining vertices with red and blue, there is always a monochromatic path connecting two of the four auxiliary points. For the coloring below, there is a red path from W to E.

Proof: Assume, for the sake of contradiction, that some coloring does not form any red path from W to E or blue path from N to S. First, for every red vertex unreachable by any red path starting from W, we change it to green. Similarly, for every blue vertex unreachable by any blue path starting from N, we change it to green, too.

Under the assumption that there is no red path from W to E and no blue path from N to S, the vertices E ans S are both changed to green. Now we have a triangulated planar graph whose four vertices on the external face, W, N, E, S are colored by red, blue, green, green respectively.

Lemma: Given a 3-coloring on a triangulated planar graph whose four vertices on the external face, say W, N, E, S, are colored by red, blue, green, green respectively. There is an internal triangular face whose vertices are of different color.

By this lemma, we can conclude that there exists an internal triangular face, say ABC, such that A is red, B blue and C green. This implies that there are a red path from W to A and a blue path from N to B. If the vertex C is originally red, then it should still be red because there is an edge between A and C. By the same reason, if it is originally blue, then it should still be blue. This gives a contradiction! To finish the proof for the Hex Game, we only need to prove the lemma.

Proof of Lemma: For each face of the triangulated planar graph, we put a vertex in it. Two vertices are adjacent if and only if their corresponding faces share a common edge whose endpoints are colored red and blue.

Note that the degree of the vertex in the external face is 1. By the handshaking lemma, there is a vertex, say $$v$$ in the internal face, say $$F$$, whose degree is odd. It is easy to see the only possible coloring of the vertices of $$F$$ to get an odd degree of $$v$$ is to color them by 3 different colors. Q.E.D.

Now I am going to show something new. The idea to consider another game board which looks like:

The idea is to get a symmetric board. To build such a game board, one can start with a hexagon in the center and then add several layers of hexagons around it. In the picture above, the hexagon labeled by 1 is the one we start with. We call it the 1st layer. Then we can add 6 hexagons labeled by 2 around it to form the 2nd layer and another 12 hexagons labeled by 3 around the 2nd layer. In general, to build a symmetric Hex of size $$n$$, one only needs to repeat the process $$n$$ times.

For this symmetric Hex game board, we have three pairs of opposite sides. One reasonable goal, which is analogous to the goal of the Hex Game, is to build a monochromatic path that links two opposite sides. However two players can cooperate to tie the game in this case.

Therefore, for the symmetric Hex, the goal is to build a monochromatic connected component that touches two opposite sides or three nonadjacent sides. For simplicity, we can such a monochromatic connected component a winning component. In other words, if we label six sides by 1 through 6, both players want to use their own colors to connect side 1 and 4, or side 2 and 5, or side 3 and 6, or side 1, 3 and 5, or side 2, 4 and 6. Again, we will only consider its dual variant in the sense that each player colors the vertex of the following graph.

Proposition: The Symmetric Hex Game can never end in a tie.

Proof: Assume, for the sake of contradiction, that there is a 2-coloring of the symmetric Hex that contains no winning component. Consider the graph with 4 extra vertices W, E, N, S which connect to the vertices of the original graph as follows.

To be specific, W connects to all the vertices on side 5, E the vertices on side 2, N the vertices on side 1 and 6, S the vertices on side 3 and 4.

By the same argument we used in the proof of the Hex Game, there is either a red path from W to E or a blue path from N to S. In other words, we should have one of the following schematic pictures.

By our assumption, it must look like the pictures on the second row. Without loss of generality, assume that we have the first picture on the second row and the blue path connects vertices $$a$$ and $$b$$.

Now we change the way the extra 4 vertices connect to the vertices on the boundary as follows.

Again, there is either a blue path from W to E or a red path from N to S. If there is a blue path from W to E, then this path combined with the blue path from $$a$$ to $$b$$ gives us a connected component that touches two opposite sides or 3 nonadjacent sides. Therefore it has to be the case that a red path connects N to S.

To summarize, whenever we have a blue path $$P_1$$ that connects side 4 and 6, there is be a red path $$P_2$$ connecting the same sides on its right side. By symmetry, there is a blue path $$P_3$$ connecting side 4 and 6 on $$P_2$$‘s right side. We can keep going and get an infinite sequence of paths connecting side 4 and 6 with alternating colors, which is impossible! Q.E.D.

Corollary For any 2-coloring of the symmetric Hex board of size $$n$$, there is a monochromatic connected component of size at least $$2n+1$$.

Proof By the symmetric Hex theorem, there is a monochromatic connected component that touches 2 opposite sides or 3 nonadjacent sides. In either case, the size of the component is at least $$2n+1$$. Q.E.D.

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## Communicating Information through Randomness

This week Staren sent me a puzzle on WeChat. After we discovered a solution for the puzzle, I tried to backtrack the source of it. I found it appeared in the blog post Yet another prisoner puzzle by Oliver Nash. The author seemed to work at a quantitative trading company SIG at that time. Given that Staren is working at JP Morgan, I guess this is one of many brain-teasers that was circulating among the traders around year 2010. Anyway, here is a version of the puzzle that was used in that blog post.

A room contains a normal 8 by 8 chessboard together with 64 identical coins, each with one “heads” side and one “tails” side. Two prisoners are at the mercy of a typically eccentric jailer who has decided to play a game with them for their freedom. The rules are the game are as follows.

The jailer will take one of the prisoners (let us call him the “first” prisoner) with him into the aforementioned room, leaving the second prisoner outside. Inside the room the jailer will place exactly one coin on each square of the chess board, choosing to show heads or tails as he sees fit (e.g. randomly). Having done this he will then choose one square of the chess board and declare to the first prisoner that this is the “magic” square. The first prisoner must then turn over exactly one of the coins and exit the room. After the first prisoner has left the room, the second prisoner is admitted. The jailer will ask him to identify the magic square. If he is able to do this, both prisoners will be granted their freedom.

These rules are explained to both prisoners before the game begins and they are allowed some time together to discuss their strategy. Of course the question is: what strategy should the prisoners adopt?

There is an extremely simple and clever solution that uses the XOR operation. However we will detour into the realm of linear algebra and information theory and come back to the elegant one-line solution at the end of the post.

In this puzzle, it doesn’t matter that we have a 8 by 8 chessboard to place the coins. All it matters is that we have $$n=64$$ places to put the coins in. Also it doesn’t matter that the jailer declare a magic square. All it matters is that he pick a magic number from $$0, 1, \ldots, n-1$$, which corresponds to the position of the magic square, and tell the first prisoner the number.

Henceforth, we can consider the following puzzle.

The jailer chooses an $$n$$-bit binary and pick a magic number from $$1, 2, \ldots, n$$. The first prisoner flips exactly at most one bit of the binary (not necessarily the magic bit) and shows the second prisoner the altered binary. What strategy should the prisoners adopt so that the second prisoner can tell the magic number?

I have changed the puzzle a little bit. In the original puzzle, the first prisoner must flip a bit. Now we allow him to do nothing if he wants to. We shall solve this weaker version of the puzzle first and come back to the original one.

If we think of the set of all the $$n$$-bit binaries as a metric space under the Hamming distance. I claim such strategy exists if and only if one can partition all the $$n$$-bit binaries into $$n$$ groups denoted as $$A_1, \ldots, A_n$$ such that for all $$k$$, the 1-neighborhood of $$A_k$$ covers all the $$n$$-bit binaries, i.e. any $$n$$-bit binary is at most 1-bit away from some binary in $$A_k$$.

The reason is as follows. Given such partition $$A_1, \ldots, A_n$$. If the jailer chooses the initial binary $$x$$ and the magic number $$k$$, because there is some binary, say $$y$$, in $$A_k$$ such that $$x$$ and $$y$$ are at most 1 bit off, the first prisoner can turn the initial binary into $$y$$ by flipping at most 1 bit in $$x$$. The second prisoner then can take this binary $$y$$, look up the partition $$A_1, \ldots, A_n$$ and find the index of the group which contains $$y$$. Conversely, one can argue it is necessary to have such a partition of all the binaries if one has the strategy.

For some reason we will see later, we call a subset $$A$$ of $$n$$-bit binaries a covering code provided its 1-neighborhood covers all $$n$$-bit binaries. In terms of covering codes, our goal is to find $$n$$ disjoint covering codes. By the way, in information theory, people call a subset of binaries a code.

Notice that if we can find $$n+1$$ disjoint covering codes, we can find a strategy that works even if we allow the jailer to pick the magic number from $$1, 2, \ldots, n+1$$. However we can not hope for more than $$n+1$$ disjoint covering codes. Here is why. Suppose we have $$N$$ disjoint covering codes. By pigeonhole principle, there is a covering code, say $$A$$, whose size is no more than $$2^n/N$$. Note that the 1-neighborhood of a binary contains exactly $$n+1$$ binaries. Hence the 1-neighborhood of $$A$$ contains at most $$2^n\times\frac{n+1}{N}$$. In order to have the 1-neighborhood of $$A$$ to cover all the binaries, we must have $$N\leq n+1$$. Moreover, if $$N=n+1$$, all codes should have the same size $$2^n/(n+1)$$.

Let us start with small $$n$$.

• For $$n=1$$, we can find 2 disjoint covering codes $$A_1=\{0\}, A_2=\{1\}$$.
• For $$n=2$$, we can find 2 disjoint covering codes $$A_1=\{00, 11\}, A_2=\{01, 10\}$$ or $$A_1=\{00, 01\}, A_2=\{10, 11\}$$.
• For $$n=3$$, we can find 4 disjoint covering codes $$A_1=\{000,111\}, A_2=\{001,110\}, A_3=\{010,101\}, A_4=\{100, 011\}$$.
• For $$n=4$$, we can take 4 disjoint covering codes by appending 0,1 to the covering codes for $$n=3$$. These covering codes are $$A_1=\{*000,*111\}, A_2=\{*001,*110\}, A_3=\{*010,*101\}, A_4=\{*100, *011\}$$ where $$*$$ is a wild card that can be either 0 or 1.
• For $$n=5$$, it is impossible to find 5 disjoint covering codes because by pigeonhole principle one of the 5 disjoint covering codes would contain no more than $$2^5/5\sim 6$$ binaries and the 1-neighborhood of any 6-element subset will always have 2 binaries not covered. This can be checked by a computer program.

It is interesting to see that for some $$n$$ we can find $$n$$ or $$n+1$$ disjoint covering codes and for some $$n$$, for instance 5, there are no $$n$$ disjoint covering codes. The situation is quite complicated here.

Therefore we had better focus on the extremal cases in which we can find $$n+1$$ disjoint covering codes. If it is the case that $$n+1$$ disjoint covering codes exist, all the covering codes should be of the same size $$2^n/(n+1)$$ and for each covering code $$A$$, the 1-neighborhoods of two distinct point in $$A$$ do not overlap. This implies that $$n=2^m-1$$ for some $$m$$. Since we are interested in the extremal case, henceforth we always assume $$n=2^m-1$$.

Even finding one covering code is hard besides that our goal is to find $$n+1$$ many of them. However, Richard Hamming in 1950 invented the first error correction code, the Hamming code $$H$$, which has the properties:

1. it is a linear code, i.e., $$H$$ is a $$(n-m)$$-dimensional linear subspace of $$\mathbb{Z}_2^n$$, the liner space of all the $$n$$-bit binaries over the finite field $$\mathbb{Z}_2$$;
2. the minimum Hamming distance between two distinct elements in $$H$$ is exactly three.

For an introduction to Hamming code, I recommend Thirty-three Miniatures by Jiri Matousek. The 5th miniature, Error-Correcting Codes, includes detailed construction of the Hamming code.

Because of the 1st property, $$H$$ has $$2^{n-m}$$ elements. The 2nd property says that the 1-neighborhoods of any two distinct elements in $$H$$ are disjoint since $$1+1<3$$. Hence the 1-neighborhood of $$H$$ contains $$2^{n-m}(n+1)=2^{n-m}2^m=2^n$$ many elements. In other words, the Hamming code is a linear covering code!

Good! We have found a linear covering code $$H$$. By translating the Hamming code $$H$$ by $$\mathbf{e_1}=100\ldots 00, \mathbf{e_2}=010\ldots 00, \ldots, \mathbf{e_n}=000\ldots 01$$ respectively, we have $$n+1$$ covering codes in total (including $$H$$ itself) since a translate of a covering code is still a covering code. Moreover, they are disjoint from each other because of the 2nd property of Hamming code.

Great! We have found $$n+1=2^m$$ disjoint covering codes for $$n$$-bit binaries. We can take this construction and use the wild card trick to get $$n+1$$ disjoint covering codes for $$n+1$$-bit binaries. In fact, for any $$n+1$$ disjoint covering codes for $$n$$-bit binaries, say $$A_0, \ldots, A_n$$, define $$A_k^* = \{0,1\}\times A_k$$. For each $$k$$, $$A_k^*$$ is a covering code for $$n$$-bit binaries, and what’s more, they are disjoint.

Hence the weaker version of the puzzle in which the first prisoner can opt to not flip the coin is totally settled since $$8\times 8=2^6$$. The same strategy works for the stronger version of the puzzle, because if unfortunately the initial binary is an element of $$A_k^* = \{0,1\}\times A_k$$ and the magic number is also $$k$$, the first prisoner can flip the leftmost bit so that the outcome stays in $$A_k^*$$.

Practically, in order to achieve such strategy, the first prisoner needs to memorize the Hamming code. However there is an easy way to get the Hamming code as it can be characterized as follows.

The Hamming code is the solution set of the linear system $$R\mathbf{x}=\mathbf{0}$$, where the matrix $$R=[\mathbf{c_1}, \mathbf{c_2}, \ldots, \mathbf{c_n}]$$ is an $$m \times n$$ matrix whose $$k$$th column $$\mathbf{c_k}$$ is the binary expansion of $$k$$ for $$k=1,2,\ldots, n$$.

For instance, when $$m=3, n=2^m-1=7$$, the matrix $$R=\begin{bmatrix}0 & 0 & 0 & 1 & 1 & 1 & 1\\ 0 & 1 & 1 & 0 & 0 & 1 & 1\\ 1 & 0 & 1 & 0 & 1 & 0 & 1\end{bmatrix}.$$

Similarly, we can characterize $$\{0,1\}\times H$$ as the solution set of $$\begin{bmatrix}\mathbf{0} & R\end{bmatrix}\mathbf{x}=\mathbf{0}.$$ We denote $$R^*=[\mathbf{c_0}, \mathbf{c_1}, \ldots, \mathbf{c_n}]$$ as the matrix $$R$$ prepended by a zero column.

Suppose $$x_0x_1\ldots x_n$$ is in $$\{0,1\}\times H$$. This means $$\sum_{x_k\neq 0}\mathbf{c_k}=\sum_{k=0}^nx_k\mathbf{c_k}=\mathbf{0},$$ where all summations take place in $$\mathbb{Z}_2^n$$. The left hand side of the equation above can be thought as the following procedure: we take all indices of the non-zero bit in the binary, $$\{k: x_k\neq 0\}$$, write out the binary expansions of them, $$\{\mathbf{c_k}: x_k\neq 0\}$$ and then add them up bitwise. Recall that adding binaries bitwise is equivalent to the XOR operation of numbers. The number corresponding to the left hand side is $$\mathrm{XOR}_{x_k\neq 0}k$$. We call this number the XOR-signature of the binary $$x_0x_1\ldots x_n$$. Therefore the XOR-signature $$\mathrm{XOR}_{x_k\neq 0}k=0$$ is another characterization of the set $$\{0,1\}\times H$$. Similarly, a binary $$x_0x_1\ldots x_n$$ is in $$\{0,1\}\times (H+\mathbf{e_k})$$ if and only if its XOR-signature is equal to $$k$$.

All right. Now we have a convenient way to tell which covering code a binary belongs to according to its XOR-signature. How will this help the first prisoner to figure out which coin to flip? Given the initial binary $$x_0x_1\ldots x_n$$, the first prisoner can compute its XOR-signature denoted by $$s$$. He wants to flip the $$k$$th bit of the binary so as to change its XOR-signature from $$s$$ to $$s’$$ which is the magic number that the jailer has picked from $$0,1,\ldots, n(=2^m-1)$$. Note that when he flips the $$k$$th bit of the binary, he will change the XOR-signature from $$s$$ to $$s \mathrm{XOR} k$$. So $$k$$ must satisfy $$s \mathrm{XOR} k=s’$$. One can solve this equation by XOR both sides by $$s$$ and get $$k=s \mathrm{XOR} s \mathrm{XOR} k = s \mathrm{XOR} s’$$.

In other words, the first prisoner only needs to compute the XOR-signature of the initial binary, say $$s$$ and flip the $$(s\mathrm{XOR}s’)$$th bit of the binary, which will change the XOR-signature of the binary from $$s$$ to $$s’$$. Then the second prisoner comes in and compute the XOR-signature of the new binary which gives the magic number.

Pretty neat, isn’t it?

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## Number of Non-isomorphic Graphs

This expository essay is to test my understanding of the techniques used in More Bricks – More Walls?, Thirty-three Miniatures by Jiří Matoušek’s.

We shall prove the sequence $$g_n(0),\ldots, g_n({n\choose 2})$$ is unimodal, i.e., it is first nondecreasing and then, from some point on, non-increasing, where $$g_n(k)$$ is the number of non-isomorphic graphs with $$n$$ vertices and $$k$$ edges. In particular, we shall prove $$g_n(0)\leq g_n(1)\leq\ldots\leq g_n(\lfloor m/2\rfloor)=g_n(\lceil m/2\rceil)\geq\ldots\geq g_n(m-1)\geq g_n(m)$$, where $$m={n\choose 2}$$.

Throughout the article, the number of vertices, which is denoted as $$n$$, is always fixed. And $$m$$ always stands for the maximal number of edges between $$n$$ vertices.

Notice that taking the complement of graphs establishes a bijection between graphs with $$k$$ edges and graphs with $$m-k$$ edges. Moreover, two graphs are isomorphic if and only if their complement graphs are isomorphic. Thus we have $$g_n(k)=g_n(m-k)$$. Hence it is enough to show that $$g_n(k)\leq g_n(l)$$ for all $$k\leq l\leq m/2$$.

Denote $$U, V$$ be the sets of all graphs with $$k, l$$ edges on the fixed vertex set $$[n]$$ respectively. Let $$r, s$$ denote the number of non-isomorphic graphs in $$U, V$$. By our notation above, $$r=g_n(k), s=g_n(l)$$. We shall show $$r\leq s$$. The graph $$G$$ is the bipartite graph between $$U$$ and $$V$$ with $$u\sim v$$ if and only if $$u$$ is a subgraph of $$v$$.

Let $$B=(b_{uv})_{u\in U, v\in V}$$ be the bipartite adjacent matrix of $$G$$, where $$b_{uv}=1$$ if $$u$$ and $$v$$ are adjacent in $$G$$, otherwise $$0$$.

We claim the matrix $$B$$ is of full rank. As it is easy to see the size of $$U$$ is no greater than the size of $$V$$, we shall prove an equivalent statement that the matrix $$B$$ is of full row rank, that is the rows of $$B$$ are linearly independent.

Suppose not. There is a non-zero row vector $$y$$ in $$\mathbb{R}^{U}$$ such that $$yB=0$$. Notice the coordinates of $$y$$ are indexed by the elements in $$U$$. Let $$K^*\in U$$ such that $$y_{K^*}\neq 0$$.

Now we partition $$U, V$$ into $$k+1$$ according to the number of edges in the intersection of the graph with $$K^*$$: $$K_i$$ is the set of graphs in $$U$$ who share $$i$$ common edges with $$K^*$$ and $$L_j$$ is the set of graphs in $$V$$ who share $$j$$ common edges with $$K^*$$ for all $$i, j=0,\ldots, k$$. Remember the number of edges in $$K^*$$ is $$k$$ and $$K_k$$ contains only one element $$K^*$$. Also all $$K_i$$ and $$L_j$$ defined above are non-empty because of the assumption $$k<l\leq m/2$$.

Note that for any $$i, j\in \{0,\ldots, k\}$$, all vertices in $$K_i$$ have the same degree to $$L_j$$ in the bipartite graph $$G$$. This is because the ways to extend a graph with $$k$$ edges and $$i$$ edges in common with $$K^*$$ to graphs with $$l$$ edges and $$j$$ edges in common with  doesn’t specifically depend the graph we start with. Denote this number as $$d_{ij}$$ and let $$D=(d_{ij})$$ be the matrix. Apparently, $$D$$ is an upper triangular matrix[/latex] with $$d_{ii}\neq 0$$. Thus $$D$$ is non-singular. On the other hand, as $$k_i\cdot d_{ij}=\sum_{K\in K_i, L\in L_j}B_{KL}$$ where $$k_i$$ is the size of $$K_i$$, $$D=\mathrm{diag}(k_0^{-1},\ldots, k_k^{-1})FBG$$ where $$F=(f_{i, u})_{i\in\{0,\ldots, k\}, u\in U}$$ and $$G=(g_{v, j})_{v\in V, j\in\{0,\ldots, k\}}$$ are matrices with $$f_{i, u}=1$$ if and only if $$u\in K_i$$ and $$g_{v, j}=1$$ if and only if $$v\in L_j$$ otherwise $$0$$.

Let $$x$$ be a $$k+1$$ dimensional row vector such that $$x=yE$$, where $$E=(e_{ui})_{u\in U, i\in\{0,\ldots, k\}}$$ is matrix with $$e_{ui}=1$$ if $$u\in K_i$$ otherwise $$0$$. In fact, $$x_i=\sum_{u\in U}y_ue_{ui}=\sum_{K\in K_i}y_K$$. Hence $$x_k=y_{K^*}\neq 0$$ as $$K_k$$ contains only one element $$K^*$$.

Now we have $$xD=yE\mathrm{diag}(k_0^{-1},\ldots, k_k^{-1})FBG$$. However it is easy to check $$EF=\mathrm{diag}(k_0,\ldots, k_k)$$ and $$yB=0$$. So $$xD=yBG=0$$ contradicting to the non-singularity of $$D$$ as $$x\neq 0$$.

Now, according to the graph isomorphism the graphs in $$U, V$$ are classified into equivalent classes, denoted as $$U_1,\ldots, U_r, V_1, \ldots, V_s$$.

Similarly, we observe that all vertices in $$V_j$$ have the same degree to $$U_i$$ since the number of ways to restrict two isomorphic graphs to a certain class of isomorphic graphs is the same. Denote this degree as $$d_{ij}$$ as well. And again we claim the matrix $$D$$(different from the one we define before) is of full row rank where $$D=(d_{ij})_{r\times s}$$. This will finish the proof since an $$r\times s$$ matrix of full row rank implies $$r\leq s$$.

Suppose not. We have a non-zero $$r$$ dimensional row vector $$y$$ such that $$yD=0$$. Let $$x\in\mathbb{R}^U$$ be the row vector indexed by $$U$$ such that $$x_u=y_i$$ for all $$u\in U_i$$. Thus for all $$u\in V_j$$ we have $$(xB)_v=\sum_{u\in U}x_uB_{uv}=\sum_{i\in[r]}\sum_{u\in U_i}x_uB_{uv}=\sum_{i\in[r]}y_i\sum_{u\in U_i}B_{uv}=\sum_{i\in[r]}y_id_{ij}=(yD)_j=0$$. Notice $$x\neq 0$$ but $$xB=0$$ which contradicts with the non-sigularity of $$B$$.

Comment: This proof appears to me as an intricate idea similar to finding an injection. To prove one set is smaller than the other, one would just hope to find an injection from one set to the other. But this is hard in this problem because those two sets we are considering are sets of equivalent classes. On the other hand, in this problem, the inclusion mapping works just fine on the level of graphs though it breaks down on the level of equivalent classes. Thus we should come up with a delicate analysis on the bipartite graph induced by the inclusion mapping. And I think this is the motivation behind the proof.

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## Fun with Hex

According to the folklore,

The Hex game was invented by the Danish mathematician Piet Hein, who introduced it in 1942 at the Niels Bohr Institute. It was independently re-invented in 1947 by the mathematician John Nash at Princeton University. The rules are really simple. Each player has an allocated color, Red and Blue being conventional. Players take turns placing a stone of their color on a single cell within the overall playing board. The goal is to form a connected path of your stones linking the opposing sides of the board marked by your colors, before your opponent connects his or her sides in a similar fashion. The first player to complete his or her connection wins the game. The four corner hexagons each belong to both adjacent sides… The game can never end in a tie, a fact proved by John Nash: the only way to prevent your opponent from forming a connecting path is to form a path yourself. In other words, Hex is a determined game.

– Hex (board game), Wikipedia

For instance, in the following 2 by 2 board, player 1 should intend to build a connected path linking $$x_1=0, x_1=2$$, while player 2 should intend to build a connected path linking $$x_2=0, x_2=2$$.

Now, we would like to play Hex game in higher dimensions. The first question is: how can we set up the game board in the $$n$$ dimensional space? The answer all depends on your point of view. If we visualize the game board in 2 dimension as follows, we can view each cell as a cube in 3 dimensional space.

Here is a rework of the 2 dimensional board which can be easily generalized to higher dimension. Consider all unit cubes in $$\mathbb{R}^3$$ who are translation of the canonical unit cube (whose vertices are $$(0,0,0), (1,0,0), (0,1,0), (0,0,1), (1,1,0), (1,0,1), (0,1,1),(1,1,1)$$) along the vector $$(x,y,z)$$, where $$(x,y,z)\in[n]\times[n]\times\mathbb{Z}$$ ($$[n]=\{1,2,\ldots, n\}$$) and $$x+y+z=0$$. Basically, those unit cubes are the ones illustrated above. Now project all those cubes to the 2-dimensional plane $$x+y+z=0$$. This gives us the hexagon board.

For higher dimension, say $$d$$ dimension, just project all unit cubes in $$\mathbb{R}^{d+1}$$ who are translation of the $$d+1$$-dimensional canonical unit cube along the vector $$(x_0, \ldots, x_n)$$, where where $$(x_0,\ldots, x_d)\in\mathbb{Z}^{d+1}$$ and $$x_0+\ldots+x_d=0$$, to the $$d$$-dimensional hyperplane given by the equation $$x_0+\ldots+x_d=0$$.

With the help of Mathematica, each cell of 3D Hex board looks like the following.

Source code is attached here for those who want to play with it a little bit in Mathematica.

v = Orthogonalize[{{1, -1, 0, 0}, {0, 1, -1, 0}, {0, 0, 1, -1}}];
data = Flatten[Table[{{x, y, z, t}.v[[1]], {x, y, z, t}.v[[2]], {x, y, z, t}.v[[3]]}, {x, 0, 1}, {y, 0, 1}, {z, 0, 1}, {t, 0, 1}], 3];
{p, s} = TetGenConvexHull[data];
Graphics3D[GraphicsComplex[p, Polygon[s]]]
`

I forgot to mention, in the $$d$$-dimensional Hex game, we have at most $$d$$ players. Again, the game will never tie. For those who are interested beyond this mere fact, please refer to David Gale’s great exposition on this topic [DG]1.

However as a paper-and-pencil game, drawing a real Hex board is time consuming. Here is another version of Hex with an easy-to-draw game board. The vertices represent the hexagon region of the graph and the edges represent the borders. Two players take turns to color the vertices and try to form a monochromatic path from one side to the opposite.

Indeed, mathematically speaking, this is the dual diagram of the original board. Its generalization in higher dimension is easy to imagine. Define a graph whose vertex set $$V=[n]^d$$.Two distinct vertices $$u, v\in V$$ are adjacent if $$u_i-v_i\in\{0,1\}$$ for all $$i\in[d]$$ or the other way around $$v_i-u_i\in\{0,1\}$$ for all $$i\in[d]$$.

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1. David Gale, The Game of Hex and the Brouwer Fixed-Point Theorem, The American Mathematical Monthly, vol. 86, 1979, pp. 818-827 []

## 欺诈猜数游戏（上）

“欺诈猜数游戏”在两个玩家甲和乙之间进行， 游戏依赖于两个甲和乙都知道的正整数$$k$$和$$n$$。

1. 若$$n\geq 2^k$$，则乙可保证获胜；
2. 对所有充分大的整数$$k$$，存在整数$$n\geq 1.99^k$$，使得乙无法保证获胜。

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## Hall’s Theorem

In 1935, the mathematician Philip Hall discovered a criteria of a perfect matching on a bipartite graph, known as Hall’s theorem, aka marriage theorem.

Considering two sets of vertices, denoted as $latex A=\{a_1, \cdots, a_m\}$ and $latex B=\{b_1, \cdots, b_n\}$. Edges are connected between $latex a_i$ and $latex b_j$ for some pairs of $latex (i, j)$. Here, we admit multiple edges between two vertices. This graph, named as $latex G$, is called a bipartite graph. One may think the bipartite graph as a model of a marriage system. Taken set $latex A$ and set $latex B$ as boys and girls, an edge between a boy and a girl tells that they might be a couple. A natural question arises that whether there exists a perfect matching, that is, every boy can find his girl without conflicting with other boys. Put formally, a perfect matching is an injective map from $latex A$ to $latex B$ which induces a subgraph of $latex G$.

Hall’s theorem asserted that there is a perfect matching in above bipartite graph $latex G$ if and on if for every subsets $latex S$ of $latex A$, the cardinal number of $latex S$(number of the members in a set) does not exceed the number of the neighbors of $latex S$. Here, the neighbors of $latex S$ are all vertices in $latex B$ which are connected with one vertex in $latex S$.

The necessary condition apparently holds. On the other hand, the proof of the sufficient part is a little bit tricky. Challenge yourself with this problem.

Now we represent several applications of Hall’s theorem.

• Let $latex A=(a_{ij})$ be a matrix of order $latex n$ with nonnegative integers as entries, such that every row and column of $latex A$ has sum $latex l$. Then $latex A$ is the sum of $latex l$ permutation matrices.
• Suppose all vertices of a directed graph $latex G$ have the same in-degrees and out-degrees of $latex l$, a fixed positive integer. Thus the graph can be separated as $latex l$ groups of circles. Every vertex appears exactly once in each group, and each edge appears in only one circle.
• (Birkhoff-von Neumann theorem) A doubly stochastic matrix, is a square matrix of nonnegative real numbers, each of whose rows and columns sums to 1. Assertion: every doubly stochastic matrix is mere a convex combination of permutation matrices.

The first two questions are equivalent with each other if one noticed the relation between a graph and its adjacent matrix. In addition, in the first problem, we construct a bipartite graph $latex G$ consisting of $latex 2n$ vertices, $latex u_1, \cdots, u_n$ and $latex v_1, \cdots, v_n$ and $latex a_{ij}$ edges between each pair of vertices $latex u_i$ and $latex v_j$. It can be easily verified that this graph meets the condition required in Hall’s theorem through contradictory argument. Thus we get a perfect matching in this graph, which conversely represent a permutation matrix. After Eliminating the perfect matching in this graph, it become to the situation of $latex l-1$ in-degrees and out-degrees. This allows us to use mathematical induction on the parameter $latex l$.

If the entries of the matrix are all rational in the third problem, the result is reduced to the first problem. Still, we shall cover the gap between rational numbers and reals. Denote all permutation matrix as $latex M_1, \cdots, M_{n!}$. For each matrix $latex (a_{ij})$ meets the requirement that the sum of each row or each column is 1, one can approach it by $latex (a_{ij}^n)$ with rational entries which is also doubly stochastic. Thus we can decompose the rational matrix $latex (a_{ij}^n)$ into a convex combination of permutation matrices with rational coefficient $latex c_k^n$, where $latex k \in \{1, \cdots, n!\}$. By finding convergent subsequence in each $latex (c_k^n)_{n=1}^\infty$, one can require that all these subsequences are convergent for the same indices, and they converge to a limit $latex c_k$. After all, the original matrix $latex (a_{ij})$ can be expressed as a convex combination of permutation matrices with coefficients $latex c_k$.

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