Turan Problems and Shadows II: Trees

Abstract: The expansion $G^+$ of a graph $G$ is the 3-uniform hypergraph obtained from $G$ by enlarging each edge of $G$ with a vertex disjoint from $V(G)$ such that distinct edges are enlarged by distinct vertices. Let ex$_r(n,F)$ denote the maximum number of edges in an $r$-uniform hypergraph with $n$ vertices not containing any copy of $F$. The authors \cite{KMV} recently determined ex$_3(n,G^+)$ more generally, namely when $G$ is a path or cycle, thus settling conjectures of F\"uredi-Jiang \cite{FJ} (for cycles) and F\"uredi-Jiang-Seiver \cite{FJS} (for paths). Here we continue this project by determining the asymptotics for ex$_3(n,G^+)$ when $G$ is any fixed forest. This settles a conjecture of F\"uredi \cite{Furedi}. Using our methods, we also show that for any graph $G$, either ex$_3(n,G^{+}) \leq \left(\frac{1}{2} + o(1)\right)n^2$ or ex$_3(n,G^{+}) \geq (1 + o(1))n^2,$ thereby exhibiting a jump for the Tur\'an number of expansions.