Abstract:For a family $\mathcal{F}$, it is called intersecting if $F\cap F'\neq \emptyset$ for all $F,F'\in\mathcal{F}$.
We use $\mathcal{SD}(\mathcal{F}) = \{F \triangle G : F, G \in \mathcal{F}\}$ to denote
the family of symmetric differences of $\mathcal{F}$.
In 2023, Frankl, Kiselev and Kupavskii conjectured that for any intersecting family $\mathcal{F} \subseteq \binom{[n]}{k}$ with $n > 10k$, the inequality $|\mathcal{SD}(\mathcal{F})| \le \sum_{\ell=0}^{k-1} \binom{n-1}{2\ell}$ holds. They further observed that a proof for the range $n>3k^2$ could likely be obtained via arguments similar to those in their earlier work, though no detailed derivation was given. In this paper, we establish the conjecture under the conditions $n\ge 60k^{3/2}$ and $k\ge 50$.
We also determine the extremal families, which are precisely a certain class of stars. A concentration inequality plays a central role in the proof.
From: Qifan Wang [view email]
[v1]
Thu, 18 Jun 2026 10:21:25 UTC (10 KB)
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