The extremal half-sized iterated-shadow intersection conjecture

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Let nn be even and let A⊂([n]n/2)\mathcal{A}\subset\binom{[n]}{n/2} satisfy μ(A)=1/2\mu(\mathcal{A})=1/2, where μ\mu is the uniform measure on the middle layer, Ac\mathcal{A}^c is the complement in that layer, and ∂+r\partial^{+r} denotes the rr-fold upper shadow. The extremal iterated-shadow conjecture. For every ϵ>0\epsilon>0, if r=⌈ϵn⌉r=\lceil\epsilon\sqrt n\rceil, then

μ(∂+r(A)∩∂+r(Ac))=Ω(ϵ).\mu\bigl(\partial^{+r}(\mathcal{A})\cap\partial^{+r}(\mathcal{A}^c)\bigr)=\Omega(\epsilon).

The claim proposes that the balanced example based on the threshold ∣A∩[n/2]∣>n/4|A\cap[n/2]|>n/4 is extremal for Friedgut's problem. The source presents it as an open problem and gives no resolution.

References

Primary source

Hou Tin Chau, David Ellis and Marius Tiba, “Intersections of iterated shadows”, arXiv:2409.05487 (2024).

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