English

Non-intersecting Ryser hypergraphs

Combinatorics 2019-10-30 v3

Abstract

A famous conjecture of Ryser states that every rr-partite hypergraph has vertex cover number at most r1r - 1 times the matching number. In recent years, hypergraphs meeting this conjectured bound, known as rr-Ryser hypergraphs, have been studied extensively. It was recently proved by Haxell, Narins and Szab\'{o} that all 33-Ryser hypergraphs with matching number ν>1\nu > 1 are essentially obtained by taking ν\nu disjoint copies of intersecting 33-Ryser hypergraphs. Abu-Khazneh showed that such a characterisation is false for r=4r = 4 by giving a computer generated example of a 44-Ryser hypergraph with ν=2\nu = 2 whose vertex set cannot be partitioned into two sets such that we have an intersecting 44-Ryser hypergraph on each of these parts. Here we construct new infinite families of rr-Ryser hypergraphs, for any given matching number ν>1\nu > 1, that do not contain two vertex disjoint intersecting rr-Ryser subhypergraphs.

Keywords

Cite

@article{arxiv.1809.06931,
  title  = {Non-intersecting Ryser hypergraphs},
  author = {Anurag Bishnoi and Valentina Pepe},
  journal= {arXiv preprint arXiv:1809.06931},
  year   = {2019}
}

Comments

8 pages, some corrections in the proof of Lemma 3.6, added more explanation in the appendix, and other minor changes