Frankl--Wang conjecture for non-trivial cross-intersecting families of different uniformities

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Let n≥2a≥2b≥4n\ge 2a\ge 2b\ge 4, and let A0∈([2,n]a)A_0\in\binom{[2,n]}{a} and B0∈([2,n]b)B_0\in\binom{[2,n]}{b} satisfy A0∩B0≠∅A_0\cap B_0\ne\emptyset. Define

A0={A0}∪{A∈([n]a):1∈A, A∩B0≠∅},\mathcal{A}_0=\{A_0\}\cup\{A\in\binom{[n]}{a}:1\in A,\ A\cap B_0\ne\emptyset\}, B0={B0}∪{B∈([n]b):1∈B, B∩A0≠∅}.\mathcal{B}_0=\{B_0\}\cup\{B\in\binom{[n]}{b}:1\in B,\ B\cap A_0\ne\emptyset\}.

Two families are cross-intersecting if every member of one intersects every member of the other, and are non-trivial if neither family is a star. Frankl--Wang's conjecture. If A⊂([n]a)\mathcal{A}\subset\binom{[n]}{a} and B⊂([n]b)\mathcal{B}\subset\binom{[n]}{b} are non-trivial cross-intersecting families, then

∣A∣∣B∣≤∣A0∣∣B0∣.|\mathcal{A}||\mathcal{B}|\le |\mathcal{A}_0||\mathcal{B}_0|.

This proposes the cross-Hilton--Milner-type pair as the extremal construction when the two families have different uniformities. The supplied source does not state whether the conjecture has been resolved.

References

Primary source

Yang Huang, “A sharp product bound for non-trivial cross-intersecting families”, arXiv:2606.23322 (2026).

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