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arXiv · 2501.06731

A note on the maximum diversity of intersecting families in the symmetric group

Jian Wang·Jimeng Xiao
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Abstract

Let $\mathcal{S}_n$ be the symmetric group on the set $[n]:=\{1,2,\ldots,n\}$. A family $\mathcal{F}\subset \mathcal{S}_n$ is called intersecting if for every $\sigma,\pi\in \mathcal{F}$ there exists some $i\in [n]$ such that $\sigma(i)=\pi(i)$. Deza and Frankl proved that the largest intersecting family of permutations is the full star, that is, the collection of all permutations with a fixed position. The diversity of an intersecting family $\mathcal{F}$ is defined as the minimum number of permutations in $\mathcal{F}$, whose deletion results in a star. In the present paper, by applying the spread approximation method developed recently by Kupavskii and Zakharov, we prove that for $n\geq 500$ the diversity of an intersecting subfamily of $\mathcal{S}_n$ is at most $(n-3)(n-3)!$, which is best possible.

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Jian Wang, Jimeng Xiao. 2025-01-12. A note on the maximum diversity of intersecting families in the symmetric group. https://doi.org/10.1016/j.ejc.2025.104331

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