Phase transitions in the Ramsey-Tur\'an theory

Abstract

Let $f(n)$ be a function and $L$ be a graph. Denote by $RT(n,L,f(n))$ the maximum number of edges of an $L$-free graph on $n$ vertices with independence number less than $f(n)$. Erd\H os and S\'os asked if $RT\left(n, K_5, c\sqrt{n}\right) = o(n^2)$ for some constant $c$. We answer this question by proving the stronger $RT\left(n, K_5, o\left(\sqrt{n\log n}\right)\right) = o(n^2)$. It is known that $RT \left(n, K_5, c \sqrt{n\log n} \right) = n^2/4+o(n^2)$ for $c>1$, so one can say that $K_5$ has a Ramsey-Tur\'an phase transition at $c\sqrt{n\log n}$. We extend this result to several other $K_s$'s and functions $f(n)$, determining many more phase transitions. We shall formulate several open problems, in particular, whether variants of the Bollob\'as-Erd\H os graph exist to give good lower bounds on $RT\left(n, K_s, f(n)\right)$ for various pairs of $s$ and $f(n)$. Among others, we use Szemer\'edi's Regularity Lemma and the Hypergraph Dependent Random Choice Lemma. We also present a short proof of the fact that $K_s$-free graphs with small independence number are sparse.