Stability for the Erd\H{o}s-Rothschild problem

TL;DR

This paper establishes a general stability theorem for the Erd ext{"o}s-Rothschild problem, characterizing the structure of extremal graphs based on solutions to an associated optimization problem, extending previous results and covering all cases with two colours.

Contribution

It provides a sufficient condition on the parameters ensuring a stability result for the problem, using a novel symmetrisation technique on edge-coloured weighted multigraphs.

Findings

A new stability theorem for the Erd ext{"o}s-Rothschild problem.

Recovery of all known stability results for s=2.

Introduction of a symmetrisation method for edge-coloured multigraphs.

Abstract

Given a sequence $\mathbf{k} := (k_1,\ldots,k_s)$ of natural numbers and a graph $G$, let $F(G;\mathbf{k})$ denote the number of colourings of the edges of $G$ with colours $1,\dots,s$ such that, for every $c \in \{1,\dots,s\}$, the edges of colour $c$ contain no clique of order $k_c$. Write $F(n;\mathbf{k})$ to denote the maximum of $F(G;\mathbf{k})$ over all graphs $G$ on $n$ vertices. This problem was first considered by Erd\H{o}s and Rothschild in 1974, but it has been solved only for a very small number of non-trivial cases. In previous work with Yilma, we constructed a finite optimisation problem whose maximum is equal to the limit of $\log_2 F(n;\mathbf{k})/{n\choose 2}$ as $n$ tends to infinity and proved a stability theorem for complete multipartite graphs $G$. In this paper we provide a sufficient condition on $\mathbf{k}$ which guarantees a general stability theorem for any graph $G$, describing the asymptotic structure of $G$ on $n$ vertices with $F(G;\mathbf{k}) = F(n;\mathbf{k}) \cdot 2^{o(n^2)}$ in terms of solutions to the optimisation problem. We apply our theorem to systematically recover existing stability results as well as all cases with $s=2$. The proof uses a novel version of symmetrisation on edge-coloured weighted multigraphs.