Supersaturated sparse graphs and hypergraphs

TL;DR

This paper investigates the enumeration of bipartite and hypergraph-free graphs, establishing bounds based on supersaturation principles and hypergraph container methods, advancing understanding in extremal combinatorics.

Contribution

It provides the first general upper bounds on the number of bipartite $H$-free graphs and hypergraphs, linking growth rates of extremal functions to enumeration.

Findings

Bound of 2^{O(ex(n,H))} for bipartite H-free graphs

Extension of supersaturation results to hypergraphs

Unified framework for known and new estimates

Abstract

A central problem in extremal graph theory is to estimate, for a given graph $H$, the number of $H$-free graphs on a given set of $n$ vertices. In the case when $H$ is not bipartite, fairly precise estimates on this number are known. In particular, thirty years ago, Erd\H{o}s, Frankl, and R\"odl proved that there are $2^{(1+o(1))\text{ex}(n,H)}$ such graphs. In the bipartite case, however, nontrivial bounds have been proven only for relatively few special graphs $H$. We make a first attempt at addressing this enumeration problem for a general bipartite graph $H$. We show that an upper bound of $2^{O(\text{ex}(n,H))}$ on the number of $H$-free graphs with $n$ vertices follows merely from a rather natural assumption on the growth rate of $n \mapsto \text{ex}(n,H)$; an analogous statement remains true when $H$ is a uniform hypergraph. Subsequently, we derive several new results, along with most previously known estimates, as simple corollaries of our theorem. At the heart of our proof lies a general supersaturation statement that extends the seminal work of Erd\H{o}s and Simonovits. The bounds on the number of $H$-free hypergraphs are derived from it using the method of hypergraph containers.