Mixed-uniformity product Hilton–Milner conjecture

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Let n2k>24n\geq 2k>2\ell\geq 4. Fix sets F0([2,n]k)F_0\in\binom{[2,n]}{k} and G0([2,n])G_0\in\binom{[2,n]}{\ell} with F0G0F_0\cap G_0\neq\emptyset, and define

F0={F0}{F([n]k):1F, FG0},\mathcal{F}_0=\{F_0\}\cup\{F\in\binom{[n]}{k}:1\in F,\ F\cap G_0\neq\emptyset\}, G0={G0}{G([n]):1G, GF0}.\mathcal{G}_0=\{G_0\}\cup\{G\in\binom{[n]}{\ell}:1\in G,\ G\cap F_0\neq\emptyset\}.

Two families are cross-intersecting if every member of one intersects every member of the other, and a family is non-trivial if the intersection of all its members is empty. Mixed-uniformity product Hilton–Milner conjecture. Suppose that F([n]k)\mathcal{F}\subset\binom{[n]}{k} and G([n])\mathcal{G}\subset\binom{[n]}{\ell} are non-trivial cross-intersecting families. Then

FGF0G0.|\mathcal{F}||\mathcal{G}|\leq |\mathcal{F}_0||\mathcal{G}_0|.

The conjecture asks for the extremal product when the two cross-intersecting families have different uniformities, with the displayed pair as the proposed extremal construction. The supplied text does not state a resolution, so the conjecture remains open.

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Sources & referencesView supporting material

Primary source

Peter Frankl and Jian Wang, “A product version of the Hilton-Milner Theorem II”, arXiv:2605.09246 (2026).

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