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

Combinatorics 2026-01-05 v2

Abstract

Let Sn\mathcal{S}_n be the symmetric group on the set [n]:={1,2,,n}[n]:=\{1,2,\ldots,n\}. A family FSn\mathcal{F}\subset \mathcal{S}_n is called intersecting if for every σ,πF\sigma,\pi\in \mathcal{F} there exists some i[n]i\in [n] such that σ(i)=π(i)\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 F\mathcal{F} is defined as the minimum number of permutations in F\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 n500n\geq 500 the diversity of an intersecting subfamily of Sn\mathcal{S}_n is at most (n3)(n3)!(n-3)(n-3)!, which is best possible.

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Cite

@article{arxiv.2501.06731,
  title  = {A note on the maximum diversity of intersecting families in the symmetric group},
  author = {Jian Wang and Jimeng Xiao},
  journal= {arXiv preprint arXiv:2501.06731},
  year   = {2026}
}

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