The equality characterization for the biased cross-intersection theorem

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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 j≥0j\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={F∈2[n]:i∈F}.\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.

References

Primary source

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

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