On Ryser's Conjecture for Linear Intersecting Multipartite Hypergraphs

Abstract

Ryser conjectured that $\tau\le(r-1)\nu$ for $r$-partite hypergraphs, where $\tau$ is the covering number and $\nu$ is the matching number. We prove this conjecture for $r\le9$ in the special case of linear intersecting hypergraphs, in other words where every pair of lines meets in exactly one vertex. Aharoni formulated a stronger version of Ryser's conjecture which specified that each $r$-partite hypergraph should have a cover of size $(r-1)\nu$ of a particular form. We provide a counterexample to Aharoni's conjecture with $r=13$ and $\nu=1$. We also report a number of computational results. For $r=7$, we find that there is no linear intersecting hypergraph that achieves the equality $\tau=r-1$ in Ryser's conjecture, although non-linear examples are known. We exhibit intersecting non-linear examples achieving equality for $r\in\{9,13,17\}$. Also, we find that $r=8$ is the smallest value of $r$ for which there exists a linear intersecting $r$-partite hypergraph that achieves $\tau=r-1$ and is not isomorphic to a subhypergraph of a projective plane.