A Jump in the Codegree Tur\'an Densities of Long Tight Cycles

József Balogh; Haoran Luo; Maya Sankar

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arXiv · 2602.18398

A Jump in the Codegree Tur\'an Densities of Long Tight Cycles

József Balogh·Haoran Luo·Maya Sankar

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Abstract

We study the codegree Tur\'an 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 Tur\'an 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 Tur\'an density. That is, if $\ell$ is sufficiently large and $r/\gcd(r,\ell)$ is odd, then the codegree Tur\'an 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 Tur\'an-type theorems for tight cycles and ``oriented colorings'' of the edge set of a hypergraph.

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József Balogh, Haoran Luo, Maya Sankar. 2026-02-20. A Jump in the Codegree Tur\'an Densities of Long Tight Cycles. https://arxiv.org/abs/2602.18398

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