Mixed-uniformity product Hilton–Milner conjecture

Let n≥2k>2ℓ≥4n\geq 2k>2\ell\geq 4. Fix sets F0∈([2,n]k)F_0\in\binom{[2,n]}{k} and G0∈([2,n]ℓ)G_0\in\binom{[2,n]}{\ell} with F0∩G0≠∅F_0\cap G_0\neq\emptyset, and define

F0={F0}∪{F∈([n]k):1∈F, F∩G0≠∅},\mathcal{F}_0=\{F_0\}\cup\{F\in\binom{[n]}{k}:1\in F,\ F\cap G_0\neq\emptyset\}, G0={G0}∪{G∈([n]ℓ):1∈G, G∩F0≠∅}.\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]ℓ)\mathcal{G}\subset\binom{[n]}{\ell} are non-trivial cross-intersecting families. Then

∣F∣∣G∣≤∣F0∣∣G0∣.|\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.

References

Primary source

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

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