Tur\'{a}n problems for star-path forests in hypergraphs

Abstract

An $r$-uniform hypergraph ($r$-graph) is linear if any two edges intersect at most one vertex. Let $\mathcal{F}$ be a given family of $r$-graphs. A hypergraph $H$ is called $\mathcal{F}$-free if $H$ does not contain any hypergraphs in $\mathcal{F}$. The Tur\'{a}n number ${\rm{ex}}_r(n,\mathcal{F})$ of $\mathcal{F}$ is defined as the maximum number of edges of all $\mathcal{F}$-free $r$-graphs on $n$ vertices, and the linear Tur\'{a}n number ${\rm{ex}}^{\rm{lin}}_r(n,\mathcal{F})$ of $\mathcal{F}$ is defined as the Tur\'{a}n number of $\mathcal{F}$ in linear host hypergraphs. An $r$-uniform linear path $P^r_\ell$ of length $\ell$ is an $r$-graph with edges $e_1,\cdots,e_\ell$ such that $|V(e_i)\cap V(e_j)|=1$ if $|i-j|=1$, and $V(e_i)\cap V(e_j)=\emptyset$ for $i\neq j$ otherwise. Gy\'{a}rf\'{a}s et al. [Linear Tur\'{a}n numbers of acyclic triple systems, European J. Combin., 2022, 103435] obtained an upper bound for the linear Tur\'{a}n number of $P_\ell^3$. In this paper, an upper bound for the linear Tur\'{a}n number of $P_\ell^r$ is obtained, which generalizes the result of $P_\ell^3$ to any $P_\ell^r$. Furthermore, some results for the linear Tur\'{a}n number and Tur\'{a}n number of some linear star-path forests are obtained