Asymptotics of the hypergraph bipartite Tur\'an problem

Abstract

For positive integers $s,t,r$, let $K_{s,t}^{(r)}$ denote the $r$-uniform hypergraph whose vertex set is the union of pairwise disjoint sets $X,Y_1,\dots,Y_t$, where $|X| = s$ and $|Y_1| = \dots = |Y_t| = r-1$, and whose edge set is $\{\{x\} \cup Y_i: x \in X, 1\leq i\leq t\}$. The study of the Tur\'an function of $K_{s,t}^{(r)}$ received considerable interest in recent years. Our main results are as follows. First, we show that \begin{equation} \mathrm{ex}(n,K_{s,t}^{(r)}) = O_{s,r}(t^{\frac{1}{s-1}}n^{r - \frac{1}{s-1}}) \end{equation} for all $s,t\geq 2$ and $r\geq 3$, improving the power of $n$ in the previously best bound and resolving a question of Mubayi and Verstra\"ete about the dependence of $\mathrm{ex}(n,K_{2,t}^{(3)})$ on $t$. Second, we show that this upper bound is tight when $r$ is even and $t \gg s$. This disproves a conjecture of Xu, Zhang and Ge. Third, we show that the above upper bound is not tight for $r = 3$, namely that $\mathrm{ex}(n,K_{s,t}^{(3)}) = O_{s,t}(n^{3 - \frac{1}{s-1} - \varepsilon_s})$ (for all $s\geq 3$). This indicates that the behaviour of $\mathrm{ex}(n,K_{s,t}^{(r)})$ might depend on the parity of $r$. Lastly, we prove a conjecture of Ergemlidze, Jiang and Methuku on the hypergraph analogue of the bipartite Tur\'an problem for graphs with bounded degrees on one side. Our tools include a novel twist on the dependent random choice method as well as a variant of the celebrated norm graphs constructed by Koll\'ar, R\'onyai and Szab\'o.