English

Piercing all maximum cliques in hypergraphs

Combinatorics 2026-04-24 v1 Metric Geometry

Abstract

Graphs whose maximum clique size exceeds half of the total number of vertices satisfy a classical property: the family of their maximum sized cliques can be pierced by a single vertex. This result dates back to a 1965 theorem by Hajnal. Motivated by this theorem, Jung, Keszegh, P\'alv\"olgyi, and Yuditsky recently conjectured that an analogous result should hold for hypergraphs of larger uniformity, with an appropriate constant replacing the threshold 1/21/2. In this paper we refute this conjecture in a strong form. We show that for any constant c<1c<1 and integers k3k\ge 3 and t1t\ge 1, there exist kk-uniform hypergraphs GG whose maximum clique size exceeds cV(G)c|V(G)|, yet the family of maximum size cliques of GG cannot be pierced by tt vertices. This demonstrates that no universal constant threshold guarantees bounded piercing number for maximum cliques in uniform hypergraphs. We discuss further questions concerning the relationship between clique size and piercing maximum cliques in hypergraphs, and introduce a geometric variant of the problem using Helly's Theorem.

Keywords

Cite

@article{arxiv.2604.21588,
  title  = {Piercing all maximum cliques in hypergraphs},
  author = {Andreas Holmsen and Attila Jung and Balázs Keszegh and Dániel G. Simon and Gábor Tardos},
  journal= {arXiv preprint arXiv:2604.21588},
  year   = {2026}
}

Comments

10 pages

R2 v1 2026-07-01T12:32:20.975Z