Sparse graphs with no polynomial-sized anticomplete pairs

TL;DR

This paper investigates the structure of graphs that exclude certain subgraphs, proving a conjecture for a broad class of graphs called almost-bipartite, and establishing stronger variants involving degree and anticomplete set conditions.

Contribution

It proves the conjecture of Conlon, Fox, and Sudakov for almost-bipartite graphs and extends it with stronger versions involving bounds on copies of H and product sizes of anticomplete sets.

Findings

The conjecture holds for almost-bipartite graphs.

Stronger bounds relate vertex degrees and sizes of anticomplete sets.

Variants include bounds on the number of copies of H and product sizes of anticomplete pairs.

Abstract

A graph is "$H$-free" if it has no induced subgraph isomorphic to $H$. A conjecture of Conlon, Fox and Sudakov states that for every graph $H$, there exists $s>0$ such that in every $H$-free graph with $n>1$ vertices, either some vertex has degree at least $sn$, or there are two disjoint sets of vertices, of sizes at least $sn^s$ and $sn$, anticomplete to each other. We prove this holds for a large class of graphs $H$, and we prove that something like it holds for all graphs $H$. Say $H$ is "almost-bipartite" if $H$ is triangle-free and $V(H)$ can be partitioned into a stable set and a set inducing a graph of maximum degree at most one. We prove that the conjecture above holds for when $H$ is almost-bipartite. We also prove a stronger version where instead of excluding $H$ we restrict the number of copies of $H$. We prove some variations on the conjecture, such as: for every graph $H$, there exists $s >0$ such that in every $H$-free graph with $n>1$ vertices, either some vertex has degree at least $sn$, or there are two disjoint sets $A, B$ of vertices with $|A||B| > s n^{1 + s}$, anticomplete to each other.