Three-fold cross-intersection product-measure conjecture

Let nn be a positive integer and let p23p\leq\frac23. Set p=(p,p,,p)(0,1)n{\bm p}=(p,p,\ldots,p)\in(0,1)^n. For i=1,2,3i=1,2,3, let Ai2[n]\mathcal A_i\subset 2^{[n]}. The families are 3-cross intersecting if

A1A2A3A_1\cap A_2\cap A_3\neq\emptyset

for every AiAiA_i\in\mathcal A_i. Let μp\mu_{\bm p} be the product biased measure on 2[n]2^{[n]}.

Three-fold cross-intersection product-measure conjecture. If A1,A2,A3\mathcal A_1,\mathcal A_2,\mathcal A_3 are 3-cross intersecting, then

μp(A1)μp(A2)μp(A3)p3.\mu_{\bm p}(\mathcal A_1)\mu_{\bm p}(\mathcal A_2)\mu_{\bm p}(\mathcal A_3)\leq p^3.

The text identifies this as an open problem and describes it as the easiest unresolved case for three cross-intersecting families; almost nothing is known for r3r\geq3.

Sources & referencesView supporting material

Primary source

Norihide Tokushige, “Application of hypergraph Hoffman's bound to intersecting families”, arXiv:2112.07965 (2021).

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