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

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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 F∩F′≠∅F\cap F'\neq\emptyset for all F,F′∈FF,F'\in\mathcal{F}. For sets F,GF,G, write F△G=(F∪G)∖(F∩G)F\triangle G=(F\cup G)\setminus(F\cap G), and define

SD(F)={F△G:F,G∈F}.\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)∣≤∑ℓ=0k−1(n−12ℓ).|\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.

References

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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