Archive for November 18th, 2015

Entropy and Sidorenko’s conjecture — after Szegedy

November 18, 2015

Here is a simple but important fact about bipartite graphs. Let G be a bipartite graph with (finite) vertex sets X and Y and edge density \alpha (meaning that the number of edges is \alpha |X||Y|). Now choose (x_1,x_2) uniformly at random from X^2 and (y_1,y_2) uniformly at random from Y^2. Then the probability that all of x_1y_1, x_1y_2, x_2y_1 and x_2y_2 are edges is at least \alpha^4.

The proof is very simple. For each x, let f_x:Y\to\{0,1\} be the characteristic function of the neighbourhood of x. That is, f_x(y)=1 if xy is an edge and 0 otherwise. Then \sum_{x,y}f_x(y) is the sum of the degrees of the x\in X, which is the number of edges of G, which is \alpha |X||Y|. If we set f=\sum_{x\in X}f_x, then this tells us that \sum_{y\in Y}f(y)=\alpha|X||Y|. By the Cauchy-Schwarz inequality, it follows that \sum_{y\in Y}f(y)^2\geq\alpha^2|X|^2|Y|.

But by the Cauchy-Schwarz inequality again,
(\sum_{y\in Y}f(y)^2)^2=(\sum_{y\in Y}\sum_{x_1,x_2\in X}f_{x_1}(y)f_{x_2}(y))^2
= (\sum_{x_1,x_2\in X}\sum_{y\in Y}f_{x_1}(y)f_{x_2}(y))^2
\leq |X|^2\sum_{x_1,x_2\in X}(\sum_{y\in Y}f_{x_1}(y)f_{x_2}(y))^2
=|X|^2\sum_{x_1,x_2\in X}\sum_{y_1,y_2\in Y}f_{x_1}(y_1)f_{x_2}(y_1)f_{x_1}(y_2)f_{x_2}(y_2).
That last expression is |X|^2 times the number of quadruples x_1,x_2,y_1,y_2 such that all of x_1y_1, x_1y_2, x_2y_1 and x_2y_2 are edges, and our previous estimate shows that it is at least \alpha^4|X|^4|Y|^2. Therefore, the probability that a random such quadruple consists entirely of edges is at least \alpha^4, as claimed (since there are |X|^2|Y|^2 possible quadruples to choose from). (more…)