Friedgut's iterated-shadow intersection conjecture

Let nn be even, let ([n]n/2)\binom{[n]}{n/2} denote the middle layer of the Boolean cube, and let A([n]n/2)\mathcal{A}\subset\binom{[n]}{n/2}. Write Ac\mathcal{A}^c for its complement in this layer, let +r(A)\partial^{+r}(\mathcal{A}) denote the rr-fold upper shadow, and let μ\mu be the uniform measure on the relevant layer. Friedgut's conjecture. For any ζ>0\zeta>0 and ϵ>0\epsilon>0, there exists δ>0\delta>0 such that, if ζμ(A)1ζ\zeta\leq\mu(\mathcal{A})\leq1-\zeta and r=ϵnr=\lceil\epsilon\sqrt{n}\rceil, then

μ(+r(A)+r(Ac))δ.\mu\bigl(\partial^{+r}(\mathcal{A})\cap\partial^{+r}(\mathcal{A}^c)\bigr)\geq\delta.

The conjecture asserts that sets of uniformly nontrivial measure have iterated upper shadows with a quantitatively positive intersection after only order-n\sqrt n iterations. It was attributed in the source to Friedgut in personal communication; no resolution is given.

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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