Frankl–Kiselev–Kupavskii conjecture on symmetric differences of intersecting families

From papers

Let [n]={1,2,,n}[n]=\{1,2,\dots,n\}, and let ([n]k)\binom{[n]}{k} denote the family of all kk-element subsets of [n][n]. A family F([n]k)\mathcal{F}\subseteq\binom{[n]}{k} is intersecting if FFF\cap F'\neq\emptyset for all F,FFF,F'\in\mathcal{F}. For sets F,GF,G, write FG=(FG)(FG)F\triangle G=(F\cup G)\setminus(F\cap G), and define

SD(F)={FG:F,GF}.\mathcal{SD}(\mathcal{F})=\{F\triangle G:F,G\in\mathcal{F}\}.

Frankl–Kiselev–Kupavskii conjecture. If F([n]k)\mathcal{F}\subseteq\binom{[n]}{k} is intersecting and n>10kn>10k, then

SD(F)=0k1(n12).|\mathcal{SD}(\mathcal{F})|\le\sum_{\ell=0}^{k-1}\binom{n-1}{2\ell}.

The right-hand side is attained by a full star, so the conjecture asserts that full stars maximize the number of symmetric differences among intersecting kk-uniform families in the stated range. The supplied text gives the conjecture's origin but no resolution, so its status is open.

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Sources & referencesView supporting material

Primary source

Lihua Feng, Zejun Huang, Qifan Wang and Yongjiang Wu, “Improved bound on symmetric differences of intersecting families”, arXiv:2606.20043 (2026).

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