Induced subgraphs of $K_r$-free graphs and the Erdős--Rogers problem

Abstract

For two graphs $F,H$ and a positive integer $n$, the function $f_{F,H}(n)$ denotes the largest $m$ such that every $H$-free graph on $n$ vertices contains an $F$-free induced subgraph on $m$ vertices. This function has been extensively studied in the last 60 years when $F$ and $H$ are cliques and became known as the Erdős-Rogers function. Recently, Balogh, Chen and Luo, and Mubayi and Verstraëte initiated the systematic study of this function in the case where $F$ is a general graph. Answering, in a strong form, a question of Mubayi and Verstraëte, we prove that for every positive integer $r$ and every $K_{r-1}$-free graph $F$, there exists some ${\epsilon }_F>0$ such that $f_{F,K_r}(n)=O(n^{1/2-{\epsilon }_F})$. This result is tight in two ways. Firstly, it is no longer true if $F$ contains $K_{r-1}$ as a subgraph. Secondly, we show that for all $r\ge 4$ and $\epsilon >0$, there exists a $K_{r-1}$-free graph $F$ for which $f_{F,K_r}(n)=\Omega (n^{1/2-\epsilon })$. Along the way of proving this, we show in particular that for every graph $F$ with minimum degree $t$, we have $f_{F,K_4}(n)=\Omega (n^{1/2-6/\sqrt{t}})$. This answers (in a strong form) another question of Mubayi and Verstraëte. Finally, we prove that there exist absolute constants $0<c<C$ such that for each $r\ge 4$, if $F$ is a bipartite graph with sufficiently large minimum degree, then $\Omega (n^{\frac{c}{\log r}})\le f_{F,K_r}(n)\le O(n^{\frac{C}{\log r}})$. This shows that for graphs $F$ with large minimum degree, the behaviour of $f_{F,K_r}(n)$ is drastically different from that of the corresponding off-diagonal Ramsey number $f_{K_2,K_r}(n)$.