Stability for the Erdős-Rothschild problem

Pikhurko, Oleg and Staden, Katherine (2023). Stability for the Erdős-Rothschild problem. Forum of Mathematics, Sigma, 11, article no. e23.



Given a sequence $\bm{k} := (k_1,\ldots,k_s)$ of natural numbers and a graph $G$, let $F(G;\bm{k})$ 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 $F(n;\bm{k})$ to denote the maximum of $F(G;\bm{k})$ 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 $\log_2 F(n;\bm{k})/{n\choose 2}$ as $n$ tends to infinity and proved a stability theorem for complete multipartite graphs $G$.

In this paper we provide a sufficient condition on $\bm{k}$ which guarantees a general stability theorem for \emph{any} graph $G$, describing the asymptotic structure of $G$ on $n$ vertices with $F(G;\bm{k}) = F(n;\bm{k}) \cdot 2^{o(n^2)}$ 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 version of symmetrisation on edge-coloured weighted multigraphs.

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