English

Generalizations and strengthenings of Ryser's conjecture

Combinatorics 2021-11-05 v3

Abstract

Ryser's conjecture says that for every rr-partite hypergraph HH with matching number ν(H)\nu(H), the vertex cover number is at most (r1)ν(H)(r-1)\nu(H). This far reaching generalization of K\"onig's theorem is only known to be true for r3r\leq 3, or ν(G)=1\nu(G)=1 and r5r\leq 5. An equivalent formulation of Ryser's conjecture is that in every rr-edge coloring of a graph GG with independence number α(G)\alpha(G), there exists at most (r1)α(G)(r-1)\alpha(G) monochromatic connected subgraphs which cover the vertex set of GG. We make the case that this latter formulation of Ryser's conjecture naturally leads to a variety of stronger conjectures and generalizations to hypergraphs and multipartite graphs. Regarding these generalizations and strengthenings, we survey the known results, improving upon some, and we introduce a collection of new problems and results.

Keywords

Cite

@article{arxiv.2009.07239,
  title  = {Generalizations and strengthenings of Ryser's conjecture},
  author = {Louis DeBiasio and Yigal Kamel and Grace McCourt and Hannah Sheats},
  journal= {arXiv preprint arXiv:2009.07239},
  year   = {2021}
}

Comments

51 pages, 9 figures, 3 tables; v3: Updated in response to referee comments; v2: Fixed minor typos and updated/reorganized material in final section