The Tur\'{a}n density of short tight cycles

TL;DR

This paper determines the maximum edge density of certain 3-uniform hypergraphs avoiding specific tight cycles, extending previous results to smaller cycle lengths and providing stability and structural insights.

Contribution

It establishes the Turán density for small tight cycles in 3-graphs and proves stability and structural theorems extending prior large-cycle results.

Findings

Turán density of 2√3 - 3 for specified tight cycles

Stability results showing near-extremal graphs are close to a 2-part construction

Maximum edges in small cycle-free 3-graphs determined up to an additive constant

Abstract

The $3$-uniform tight $\ell$-cycle $C_\ell^{3}$ is the $3$-graph on $\{1,\dots,\ell\}$ consisting of all $\ell$ consecutive triples in the cyclic order. Let $\mathcal{C}$ be either the pair $\{C_{4}^{3}, C_{5}^{3}\}$ or the single tight $\ell$-cycle $C_{\ell}^{3}$ for some $\ell\ge 7$ not divisible by $3$. We show that the Tur\'an density of $\mathcal{C}$, that is, the asymptotically maximal edge density of a large $\mathcal{C}$-free $3$-graph, is equal to $2\sqrt{3} - 3$. We also establish the corresponding Erd\H{o}s-Simonovits-type stability result, informally stating that all almost maximum $\mathcal{C}$-free graphs are close in the edit distance to a 2-part recursive construction. This extends the earlier analogous results of Kam\v{c}ev-Letzter-Pokrovskiy ["The Tur\'an density of tight cycles in three-uniform hypergraphs", Int. Math. Res. Not. 6 (2024), 4804-4841] that apply for sufficiently large $\ell$ only. Additionally, we prove a finer structural result that allows us to determine the maximum number of edges in a $\{C_{4}^{3}, C_{5}^{3}\}$-free $3$-graph with a given number of vertices up to an additive $O(1)$ error term.