Generalizations and strengthenings of Ryser's conjecture

Abstract

Ryser's conjecture says that for an r-partite hypergraph H with matching number ν(H), the vertex cover number is at most (r − 1)ν(H). This far reaching generalization of König's theorem is only known to be true for r ≤ 3, or α(G) = 1 and r ≤ 5. An equivalent formulation of Ryser's conjecture is that in every r-edge coloring of a graph G with independence number α(G), there exists at most (r − 1)α(G) monochromatic connected subgraphs which cover the vertex set of G.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. In regards to these generalizations and strengthenings, we survey the known results, improving upon some, and we introduce a collection of new problems and results.