The density Tur\'an problem for hypergraphs

Abstract

Given a $k$-graph $H$ a complete blow-up of $H$ is a $k$-graph $\hat{H}$ formed by replacing each $v\in V(H)$ by a non-empty vertex class $A_v$ and then inserting all edges between any $k$ vertex classes corresponding to an edge of $H$. Given a subgraph $G\subseteq \hat{H}$ and an edge $e\in E(H)$ we define the density $d_e(G)$ to be the proportion of edges present in $G$ between the classes corresponding to $e$. The density Tur\'an problem for $H$ asks: determine the minimal value $d_{crit}(H)$ such that any subgraph $G\subseteq \hat{H}$ satisfying $d_e(G)> d_{crit}(H)$ for every $e\in E(H)$ contains a copy of $H$ as a transversal, i.e. a copy of $H$ meeting each vertex class of $\hat{H}$ exactly once. We give upper bounds for this hypergraph density Tur\'an problem that generalise the known bounds for the case of graphs due to Csikv\'ari and Nagy, [Combinatorics, Probability and Computing, 21(4):531-553, 2012] although our methods are different, employing an entropy compression argument.