Multicolor Tur\'an numbers II -- a generalization of the Ruzsa-Szemer\'edi theorem and new results on cliques and odd cycles

Abstract

In this paper we continue the study of a natural generalization of Tur\'an's forbidden subgraph problem and the Ruzsa-Szemer\'edi problem. Let $ex_F(n,G)$ denote the maximum number of edge-disjoint copies of a fixed simple graph $F$ that can be placed on an $n$-vertex ground set without forming a subgraph $G$ whose edges are from different $F$-copies. The case when both $F$ and $G$ are triangles essentially gives back the theorem of Ruzsa and Szemer\'edi. We extend their results to the case when $F$ and $G$ are arbitrary cliques by applying a number theoretic result due to Erd\H{o}s, Frankl and R\"odl. This extension in turn decides the order of magnitude for a large family of graph pairs, which will be subquadratic, but almost quadratic. Since the linear $r$-uniform hypergraph Tur\'an problems to determine $ex_r^{lin}(n,G)$ form a class of the multicolor Tur\'an problem, following the identity $ex_r^{lin}(n,G)=ex_{K_r}(n,G)$, our results determine the linear hypergraph Tur\'an numbers of every graph of girth $3$ and for every $r$ up to a subpolynomial factor. Furthermore, when $G$ is a triangle, we settle the case $F=C_5$ and give bounds for the cases $F=C_{2k+1}$, $k\ge 3$ as well.