The typical structure of Gallai colorings and their extremal graphs

Abstract

An edge coloring of a graph $G$ is a Gallai coloring if it contains no rainbow triangle. We show that the number of Gallai $r$-colorings of $K_n$ is $\left(\left(\genfrac{}{}{0ex}{}{r}{2}\right)+o(1)\right)2^{\left(\genfrac{}{}{0ex}{}{n}{2}\right)}$. This result indicates that almost all Gallai $r$-colorings of $K_n$ use only 2 colors. We also study the extremal behavior of Gallai $r$-colorings among all $n$-vertex graphs. We prove that the complete graph $K_n$ admits the largest number of Gallai $3$-colorings among all $n$-vertex graphs when $n$ is sufficiently large, while for $r\ge 4$, it is the complete bipartite graph $K_{\lfloor n/2\rfloor ,\lceil n/2\rceil }$. Our main approach is based on the hypergraph container method, developed independently by Balogh, Morris, and Samotij as well as by Saxton and Thomason, together with some stability results for containers.

28 pages