RELATIVE TURÁN PROBLEMS FOR UNIFORM HYPERGRAPHS.

For two graphs $F$ and $H$, the relative Turán number ${ex}(H,F)$ is the maximum number of edges in an $F$-free subgraph of $H$. Foucaud, Krivelevich, and Perarnau [SIAM J. Discrete Math., 29 (2015), pp. 65--78] and Perarnau and Reed [Combin. Probab. Comput., 26 (2017), pp. 448--467] studied these quantities as a function of the maximum degree of $H$. In this paper, we study a generalization for uniform hypergraphs. If $F$ is a complete $r$-partite $r$-uniform hypergraph with parts of sizes $s_1,s_2,\dots,s_r$ with each $s_{i + 1}$ sufficiently large relative to $s_i$, then with $1/\beta = \sum_{i = 2}^r \prod_{j = 1}^{i - 1} s_j$ we prove that for any $r$-uniform hypergraph $H$ with maximum degree $\Delta$, ${ex}(H,F)\ge \Delta^{-\beta - o(1)} \cdot e(H).$ This is tight as $\Delta \rightarrow \infty$ up to the $o(1)$ term in the exponent, since we show there exists a $\Delta$-regular $r$-graph $H$ such that ${ex}(H,F)=O(\Delta^{-\beta}) \cdot e(H)$. Similar tight results are obtained when $H$ is the random $n$-vertex $r$-graph $H_{n,p}^r$ with edge-probability $p$, extending results of Balogh and Samotij [J. Lond. Math. Soc., 83 (2011), pp. 1091--1094] and Morris and Saxton [Adv. Math., 298 (2016), pp. 534--580]. General lower bounds for a wider class of $F$ are also obtained. [ABSTRACT FROM AUTHOR]