A Jump in the Codegree Turán Densities of Long Tight Cycles

Abstract

We study the codegree Turán density of ${\mathcal{C}}_{\ell }^r$, the $r$-uniform hypergraph tight cycle of length $\ell$. A result of Han, Lo, and Sanhueza-Matamala states that if $\ell$ is sufficiently large and $r/\gcd (r,\ell )$ is even, then the codegree Turán density of ${\mathcal{C}}_{\ell }^r$ is $1/2$. We prove that whenever the latter assumption is not satisfied, there is a significant drop in the codegree Turán density. That is, if $\ell$ is sufficiently large and $r/\gcd (r,\ell )$ is odd, then the codegree Turán density of ${\mathcal{C}}_{\ell }^r$ can be at most $1/3$. Moreover, this bound is tight for infinitely many uniformities $r$ and all sufficiently large $\ell$ in the corresponding residue classes modulo $r$. Our proof makes use of a group-theoretic connection between Turán-type theorems for tight cycles and ``oriented colorings'' of the edge set of a hypergraph.

15 pages, 4 figures. Comments are welcome