The chromatic profile of locally colourable graphs

Abstract

The classical Andr\'{a}sfai-Erd\H{o}s-S\'{o}s theorem considers the chromatic number of $K_{r + 1}$-free graphs with large minimum degree, and in the case $r = 2$ says that any $n$-vertex triangle-free graph with minimum degree greater than $2/5 \cdot n$ is bipartite. This began the study of the chromatic profile of triangle-free graphs: for each $k$, what minimum degree guarantees that a triangle-free graph is $k$-colourable? The profile has been extensively studied and was finally determined by Brandt and Thomass\'{e}. Triangle-free graphs are exactly those in which each neighbourhood is one-colourable. As a natural variant, \L uczak and Thomass\'{e} introduced the notion of a locally bipartite graph in which each neighbourhood is 2-colourable. Here we study the chromatic profile of the family of graphs in which every neighbourhood is $b$-colourable (locally $b$-partite graphs) as well as the family where the common neighbourhood of every $a$-clique is $b$-colourable. Our results include the chromatic thresholds of these families as well as showing that every $n$-vertex locally $b$-partite graph with minimum degree greater than $(1 - 1/(b + 1/7)) \cdot n$ is $(b + 1)$-colourable. Understanding these locally colourable graphs is crucial for extending the Andr\'{a}sfai-Erd\H{o}s-S\'{o}s theorem to non-complete graphs, which we develop elsewhere.