Forbidding matching as trace in uniform hypergraphs

Abstract

We say a hypergraph $\mathcal{H}$ contains a hypergraph $\mathcal{G}$ as trace if there exists a vertex subset $S\subseteq V(\mathcal{H})$ such that $|S|=|V(\mathcal{G})|$ and $\{e\cap S:e\in E(\mathcal{H})\}$ contains $\mathcal{G}$ as a sub-hypergraph. We use ${\mathrm{ex}}_r(n,{\mathrm{Tr}}_r(\mathcal{G}))$ to denote the maximum number of hyperedges in an $r$-uniform hypergraph on $n$ vertices not containing $\mathcal{G}$ as a trace. The study of Turán numbers for traces was initiated by Mubayi and Zhao who studied the case when $\mathcal{G}$ is a complete graph. Let $M_{s+1}$ denote the graph of a matching with $s+1$ edges. In this paper, we give the upper bound of ${\mathrm{ex}}_r(n,{\mathrm{Tr}}_r(M_{s+1}))$ which is sharp asymptotically. When $r=3$, we give the exact value of ${\mathrm{ex}}_3(n,{\mathrm{Tr}}_3(M_{s+1}))$. We also consider the generalized Turán number in the case of matching. That is, the maximum number of copies of clique ${\mathcal{K}}_t^r$ in hypergraphs forbidding ${\mathrm{Tr}}_r(M_{s+1})$ as a trace. We give an upper bound which is sharp asymptotically and when $r=3$, we give the exact value. The Turán number of forbidding a matching and the other graph is another well studied topic initiated by Alon and Frankl. We also consider an analogue problem for the trace version, i.e., forbidding trace of matching and trace of complete graph as subgraphs.