A local clique density theorem in $H$-free graphs

Abstract

In 2016, Reiher's clique density theorem determined the minimum number of copies of $K_t$ in a graph with a prescribed edge density. In this paper, we investigate its local version and prove a local clique density theorem in $H$-free graphs as follows. For integers $r$ and $t$ with $2\leq t\leq r-1$, any $r$-chromatic graph $H$, any real numbers $γ$ and $α$ with $\frac{t-2}{2(t-1)}\leqγ\leq \frac{r-2}{2(r-1)}$ and $0\leqα\leq 1$, we determine the maximum value $β:=β(r,t,α,γ)$ such that for every $n$-vertex $H$-free graph $G$ with at least $γn^2$ edges, every $\lceilαn\rceil$-vertex subset in $G$ contains at least $(β-o(1))n^{t}$ copies of $K_t$. In particular, when $H=K_r$, every $\lceilαn\rceil$-vertex subset contains at least $\lfloorβn^t\rfloor$ copies of $K_t$, which is an exact bound. For suitable choices of $α$ and $γ$, namely, those for which all part ratios in the corresponding extremal construction are rational, this bound is attained for infinitely many values of $n$.