Generalized Turán problems for a matching and long cycles (2412.18853v1)

Abstract: Let $\mathscr{F}$ be a family of graphs. A graph $G$ is $\mathscr{F}$-free if $G$ does not contain any $F\in \mathcal{F}$ as a subgraph. The general Tur\'an number, denoted by $ex(n, H,\mathscr{F})$, is the maximum number of copies of $H$ in an $n$-vertex $\mathscr{F}$-free graph. Then $ex(n, K_2,\mathscr{F})$, also denote by $ex(n, \mathscr{F})$, is the Tur\'an number. Recently, Alon and Frankl determined the exact value of $ex(n, {K_{k},M_{s+1}})$, where $K_{k}$ and $M_{s+1}$ are a complete graph on $k $ vertices and a matching of size $s +1$, respectively. Then many results were obtained by extending $K_{k}$ to a general fixed graph or family of graphs. Let $C_k$ be a cycle of order $k$. Denote $C_{\ge k}={C_k,C_{k+1},\ldots}$. In this paper, we determine the value of $ex(n,K_r, {C_{\ge k},M_{s+1}})$ for large enough $n$ and obtain the extremal graphs when $k$ is odd. Particularly, the exact value of $ex(n, {C_{\ge k},M_{s+1}})$ and the extremal graph are given for large enough $n$.