Forbidding $r$-extension of $C_4$ -- a counterexample of the Anstee-Sali conjecture

Abstract

In this note we prove $\mathrm{forb}(n,1_{r-2}\times C_4)=\Theta (n^{r-1/2})$, thus providing a series of counterexamples of a 2005 conjecture of the first two authors. Here $1_{r-2}\times C_4$ is the $r$-extension of the 4-cycle, that is the $r$-uniform hypergraph of four edges obtained by joining a set of $r-2$ vertices to each edge of $C_4$. $\mathrm{forb}(n,F)$ is the largest number of edges of a simple hypergraph $G$ on $n$ vertices without \emph{trace} (induced subhypergraph) $F$.