Problems / combinatorics
combinatorics / Extremal hypergraph theory; covers and matchings
Ryser's conjecture on covers of r-partite hypergraphs
Ryser's conjecture is false, already for intersecting hypergraphs. September 23: for every sufficiently large prime s≡2(mod3) and every sufficiently large odd n (threshold depending on s), there is an intersecting (sn+1)-partite (sn+1)-uniform hypergraph with τ=sn+1, against the conjectured sn. September 27: for every sufficiently large prime q, an intersecting (q+1)-partite (q+1)-uniform hypergraph with τ=q+1 and exactly q+1 nonisolated vertices in each part. Not given: an explicit rank or example, or anything for small r (cases r=4,5 remain open in general).