Extremal results in sparse pseudorandom graphs

TL;DR

This paper introduces a new counting lemma for sparse pseudorandom graphs, enabling the extension of key combinatorial theorems to sparse settings, thus advancing extremal graph theory.

Contribution

It develops a novel counting lemma following Gowers' functional approach, complementing existing sparse regularity lemmas, to count small graphs in sparse pseudorandom graphs.

Findings

Proved a new counting lemma for sparse pseudorandom graphs.

Extended classical combinatorial theorems to sparse graph settings.

Improved upon previous results in sparse extremal combinatorics.

Abstract

Szemer\'edi's regularity lemma is a fundamental tool in extremal combinatorics. However, the original version is only helpful in studying dense graphs. In the 1990s, Kohayakawa and R\"odl proved an analogue of Szemer\'edi's regularity lemma for sparse graphs as part of a general program toward extending extremal results to sparse graphs. Many of the key applications of Szemer\'edi's regularity lemma use an associated counting lemma. In order to prove extensions of these results which also apply to sparse graphs, it remained a well-known open problem to prove a counting lemma in sparse graphs. The main advance of this paper lies in a new counting lemma, proved following the functional approach of Gowers, which complements the sparse regularity lemma of Kohayakawa and R\"odl, allowing us to count small graphs in regular subgraphs of a sufficiently pseudorandom graph. We use this to prove sparse extensions of several well-known combinatorial theorems, including the removal lemmas for graphs and groups, the Erd\H{o}s-Stone-Simonovits theorem and Ramsey's theorem. These results extend and improve upon a substantial body of previous work.