On an extremal problem for locally sparse multigraphs

Abstract

A multigraph $G$ is an $(s,q)$-graph if every $s$-set of vertices in $G$ supports at most $q$ edges of $G$, counting multiplicities. Mubayi and Terry posed the problem of determining the maximum of the product of the edge-multiplicities in an $(s,q)$-graph on $n$ vertices. We give an asymptotic solution to this problem for the family $(s,q)=(2r, a\binom{2r}{2}+\mathrm{ex}(2r, K_{r+1})-1 )$ with $r, a\in \mathbb{Z}_{\geq 2}$. This greatly generalises previous results on the problem due to Mubayi and Terry and to Day, Treglown and the author, who between them had resolved the special case $r=2$. Our result asymptotically confirms an infinite family of cases in (and overcomes a major obstacle to a resolution of) a conjecture of Day, Treglown and the author.