Kruskal--Katona-Type Problems via the Entropy Method

TL;DR

This paper applies the entropy method to solve extremal combinatorics problems, including a sharp bound on rainbow triangles in edge-colored graphs and a generalized Kruskal--Katona theorem, advancing bounds in combinatorial extremal problems.

Contribution

It introduces a novel entropy-based approach to Kruskal--Katona-type problems, improving existing bounds and generalizing classical theorems.

Findings

Maximum rainbow triangles in 3-edge-colored graphs is at most rom or sharpness.

Generalized Kruskal--Katona theorem encompassing previous results.

Enhanced bounds via a clever use of Shearer's inequality.

Abstract

In this paper, we investigate several extremal combinatorics problems that ask for the maximum number of copies of a fixed subgraph given the number of edges. We call problems of this type Kruskal--Katona-type problems. Most of the problems that will be discussed in this paper are related to the joints problem. There are two main results in this paper. First, we prove that, in a $3$-edge-colored graph with $R$ red, $G$ green, $B$ blue edges, the number of rainbow triangles is at most $\sqrt{2RGB}$, which is sharp. Second, we give a generalization of the Kruskal--Katona theorem that implies many other previous generalizations. Both arguments use the entropy method, and the main innovation lies in a more clever argument that improves bounds given by Shearer's inequality.