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Turan Problems and Shadows III: expansions of graphs

The expansion $G^+$ of a graph $G$ is the $3$-uniform hypergraph obtained from $G$ by enlarging each edge of $G$ with a new vertex disjoint from $V(G)$ such that distinct edges are enlarged by distinct vertices. Let $ex_3(n,F)$ denote the maximum number of edges in a $3$-uniform hypergraph with $n$ vertices not containing any copy of a $3$-uniform hypergraph $F$. The study of $ex_3(n,G^+)$ includes some well-researched problems, including the case that $F$ consists of $k$ disjoint edges, $G$ is a triangle, $G$ is a path or cycle, and $G$ is a tree. In this paper we initiate a broader study of the behavior of $ex_3(n,G^+)$. Specifically, we show \[ ex_3(n,K_{s,t}^+) = Θ(n^{3 - 3/s})\] whenever $t > (s - 1)!$ and $s \geq 3$. One of the main open problems is to determine for which graphs $G$ the quantity $ex_3(n,G^+)$ is quadratic in $n$. We show that this occurs when $G$ is any bipartite graph with Turán number $o(n^φ)$ where $φ= \frac{1 + \sqrt{5}}{2}$, and in particular, this shows $ex_3(n,Q^+) = Θ(n^2)$ where $Q$ is the three-dimensional cube graph.