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)s\equiv2\pmod3 and every sufficiently large odd nn (threshold depending on ss), there is an intersecting (sn+1)(s^n+1)-partite (sn+1)(s^n+1)-uniform hypergraph with τ=sn+1\tau=s^n+1, against the conjectured sns^n. September 27: for every sufficiently large prime qq, an intersecting (q+1)(q+1)-partite (q+1)(q+1)-uniform hypergraph with τ=q+1\tau=q+1 and exactly q+1q+1 nonisolated vertices in each part. Not given: an explicit rank or example, or anything for small rr (cases r=4,5r=4,5 remain open in general).

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combinatoricsSep 23, 2026Significance 50/100Registry: lean checked

Ryser's conjecture on covers of r-partite hypergraphs

Prior state unknown→disproved

Ryser's conjecture is false, already for intersecting hypergraphs. September 23: for every sufficiently large prime s≡2(mod3)s\equiv2\pmod3 and every sufficiently large odd nn (threshold depending on ss), there is an intersecting (sn+1)(s^n+1)-partite (sn+1)(s^n+1)-uniform hypergraph with τ=sn+1\tau=s^n+1, against the conjectured sns^n. September 27: for every sufficiently large prime qq, an intersecting (q+1)(q+1)-partite (q+1)(q+1)-uni…

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Ryser's conjecture is false, already for intersecting hypergraphs. September 23: for every sufficiently large prime s≡2(mod3)s\equiv2\pmod3 and every sufficiently large odd nn (threshold depending on ss), there is an intersecting (sn+1)(s^n+1)-partite (sn+1)(s^n+1)-uniform hypergraph with τ=sn+1\tau=s^n+1, against the conjectured sns^n. September 27: for every sufficiently large prime qq, an intersecting (q+1)(q+1)-partite (q+1)(q+1)-uniform hypergraph with τ=q+1\tau=q+1 and exactly q+1q+1 nonisolated vertices in each part. Not given: an explicit rank or example, or anything for small rr (cases r=4,5r=4,5 remain open in general).

Ryser's conjecture is false, already for intersecting hypergraphs. September 23: for every sufficiently large prime s≡2(mod3)s\equiv2\pmod3 and every sufficiently large odd nn (threshold depending on ss), there is an intersecting (sn+1)(s^n+1)-partite (sn+1)(s^n+1)-uniform hypergraph with τ=sn+1\tau=s^n+1, against the conjectured sns^n. September 27: for every sufficiently large prime qq, an intersecting (q+1)(q+1)-partite (q+1)(q+1)-uniform hypergraph with τ=q+1\tau=q+1 and exactly q+1q+1 nonisolated vertices in each part. Not given: an explicit rank or example, or anything for small rr (cases r=4,5r=4,5 remain open in general).

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Ryser's conjecture on covers of r-partite hypergraphs — Mathematical Frontier Network