English

Intersecting hypergraphs with large cover number

Combinatorics 2025-04-15 v2

Abstract

In their famous 1974 paper introducing the local lemma, Erd\H{o}s and Lov\'asz posed a question-later referred by Erd\H{o}s as one of his three favorite open problems: What is the minimum number of edges in an rr-uniform, intersecting hypergraph with cover number rr? This question was solved up to a constant factor in Kahn's remarkable 1994 paper. More recently, motivated by applications to Bollob\'as' ''power of many colours'' problem, Alon, Buci\'c, Christoph, and Krivelevich introduced a natural generalization by imposing a space constraint that limits the hypergraph to use only nn vertices. In this note we settle this question asymptotically, up to a logarithmic factor in n/rn/r in the exponent, for the entire range.

Keywords

Cite

@article{arxiv.2503.14918,
  title  = {Intersecting hypergraphs with large cover number},
  author = {Matija Bucić and Vanshika Jain and Varun Sivashankar},
  journal= {arXiv preprint arXiv:2503.14918},
  year   = {2025}
}
R2 v1 2026-06-28T22:26:17.878Z