Complete counterexamples to Mubayi's supersaturation conjecture for graphs

Abstract

For a graph $F$, its extremal number ${\rm ex}(n,F)$ and extremal graphs denote the maximum number of edges in $F$-free graphs and the graphs attaining this maximum, respectively. Let $h_F(n,q)$ be the minimum number of copies of $F$ in an $n$-vertex graph with ${\rm ex}(n,F)+q$ edges, and let $t_F(n,q)$ denote the corresponding minimum obtained by adding $q$ edges to an extremal graph. Mubayi (2013) conjectured that for every non-$r$-partite stable $r$-uniform hypergraph $F$ with $r\ge 2$, $h_F(n,q)\ge q\cdot t_F(n,1)$ holds for all sufficiently large $n$ and all integers $q\ge 1$. In this paper, we prove that Mubayi's conjecture fails in a stronger sense. For every integer $d\ge 2$, we construct a family of non-$d$-partite stable graphs $F$ such that $h_F(n,q)< q\cdot t_F(n,1)$ for all $1\le q\le O\bigl(n^{4/ 3}\bigr)$. In particular, these examplesis completely refute Mubayi's conjecture. \noindent\emph{Key words}: extremal graph; supersaturation; chromatic number.