The extremal half-sized iterated-shadow intersection conjecture

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=ϵnr=\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.

Sources & referencesView supporting material

Primary source

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

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