The shifting method and generalized Turán number of matchings

Abstract

Given two graphs $T$ and $F$, the maximum number of copies of $T$ in an $F$-free graph on $n$ vertices is called the generalized Turán number, denoted by $ex(n,T,F)$. When $T=K_2$, it reduces to the classical Turán number $ex(n,F)$. Let $M_k$ be a matching with $k$ edges and $K_{s,t}^{\ast }$ a graph obtained from $K_{s,t}$ by replacing the part of size $s$ by a clique of the same size. In this paper, we show that for any $s\ge 2$ and $n\ge 2k+1$, \[ ex(n,K_s,M_{k+1})=\max\left\{\binom{2k+1}{s}, \binom{k}{s}+(n-k)\binom{k}{s-1}\right\}. \] For any $s\ge 1$, $t\ge 2$ and $n\ge 2k+1$, \[ ex(n,K_{s,t}^*,M_{k+1})=\max\left\{\binom{2k+1}{s+t}\binom{s+t}{t}, \binom{k}{s}\binom{n-s}{t}+(n-k)\binom{k}{s+t-1}\binom{s+t-1}{t}\right\}. \] Moreover, we also study the bipartite case of the problem. Let $e{x}_{bip}(n,T,F)$ be the maximum possible number of copies of $T$ in an $F$-free bipartite graph with each part of size $n$. We prove that for any $s,t\ge 1$ and $n\ge k$, \[ ex_{bip}(n,K_{s,t},M_{k+1})=\left\{ \begin{aligned} &\binom{k}{s}\binom{n}{t}+\binom{k}{t}\binom{n}{s}, & \quad s\neq t, &\binom{k}{s}\binom{n}{s},&\quad s=t. \end{aligned} \right. \] Our proof is mainly based on the shifting method.

arXiv commentto appear in European Journal of Combinatorics