The equality characterization for the biased cross-intersection theorem

Let Ω\Omega be the parameter domain used in Theorem~, and let μα\mu_\alpha and μβ\mu_\beta denote the corresponding biased measures on 2[n]2^{[n]}. Let Aj\mathcal A_j and Bj\mathcal B_j be the families defined before that theorem. Assume that α\alpha and β\beta satisfy the premises of Theorem~, namely

μα(Aj)μβ(Bj)<αβ\mu_\alpha(\mathcal A_j)\mu_\beta(\mathcal B_j)<\alpha\beta

for every j0j\geq0. The equality characterization. If F\mathcal F and G\mathcal G are cross-intersecting families in 2[n]2^{[n]} satisfying μα(F)μβ(G)=αβ\mu_\alpha(\mathcal F)\mu_\beta(\mathcal G)=\alpha\beta, then there is some i[n]i\in[n] such that

F=G={F2[n]:iF}.\mathcal F=\mathcal G=\{F\in2^{[n]}:i\in F\}.

This is a uniqueness assertion for equality in the biased cross-intersection bound established by the preceding theorem; because the supplied text gives no indication that this conjectural equality characterization has been proved, its resolution remains open.

Sources & referencesView supporting material

Primary source

Norihide Tokushige, “When are stars the largest cross intersecting families?”, arXiv:1810.06820 (2019).

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