Counting substructures I: color critical graphs

Abstract

Let $F$ be a graph which contains an edge whose deletion reduces its chromatic number. We prove tight bounds on the number of copies of $F$ in a graph with a prescribed number of vertices and edges. Our results extend those of Simonovits, who proved that there is one copy of $F$, and of Rademacher, Erd\H os and Lov\'asz-Simonovits, who proved similar counting results when $F$ is a complete graph. One of the simplest cases of our theorem is the following new result. There is an absolute positive constant $c$ such that if $n$ is sufficiently large and $1 \le q < cn$, then every $n$ vertex graph with $n$ even and $n^2/4 +q$ edges contains at least $q(n/2)(n/2-1)(n/2-2)$ copies of a five cycle. Similar statements hold for any odd cycle and the bounds are best possible.