English

Vanishing codegree Tur\'{a}n density implies vanishing uniform Tur\'{a}n density

Combinatorics 2023-12-06 v1

Abstract

For a kk-uniform hypergraph (or simply kk-graph) FF, the codegree Tur\'{a}n density πco(F)\pi_{\mathrm{co}}(F) is the infimum over all α\alpha such that any nn-vertex kk-graph HH with every (k1)(k-1)-subset of V(H)V(H) contained in at least αn\alpha n edges has a copy of FF. The uniform Tur\'{a}n density π(F)\pi_{\therefore}(F) is the supremum over all dd such that there are infinitely many FF-free kk-graphs HH satisfying that any linear-size subhypergraph of HH has edge density at least dd. Falgas-Ravry, Pikhurko, Vaughan and Volec [J. London Math. Soc., 2023] asked whether for every 33-graph FF, π(F)πco(F)\pi_{\therefore}(F)\leq\pi_{\mathrm{co}}(F). We provide a positive answer to this question provided that πco(F)=0\pi_{\mathrm{co}}(F)=0. Our proof relies on a random geometric construction and a new formulation of the characterization of 33-graphs with vanishing uniform Tur\'{a}n density due to Reiher, R{\"o}dl and Schacht [J. London Math. Soc., 2018]. Along the way, we answer a question of Falgas-Ravry, Pikhurko, Vaughan and Volec about subhypergraphs with linear minimum codegree in uniformly dense hypergraphs in the negative.

Keywords

Cite

@article{arxiv.2312.02879,
  title  = {Vanishing codegree Tur\'{a}n density implies vanishing uniform Tur\'{a}n density},
  author = {Laihao Ding and Hong Liu and Shuaichao Wang and Haotian Yang},
  journal= {arXiv preprint arXiv:2312.02879},
  year   = {2023}
}

Comments

10 pages

R2 v1 2026-06-28T13:41:50.698Z