Ellis–Friedgut–Pilpel conjecture on extremal t-intersecting families of permutations

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Let SnS_n be the symmetric group, and let Fixed(σ)\mathrm{Fixed}(\sigma) denote the set of fixed points of σ\sigma. A family F⊂SnF\subset S_n is tt-intersecting if any two of its permutations agree on at least tt points. For n,t,rn,t,r with n∈Nn\in\mathbb N, 1≤t≤n1\leq t\leq n, define

Fn,t,r={σ∈Sn:∣Fixed(σ)∩{1,2,…,t+2r}∣≥t+r}.\mathcal F_{n,t,r}=\{\sigma\in S_n:|\mathrm{Fixed}(\sigma)\cap\{1,2,\ldots,t+2r\}|\geq t+r\}.

Ellis–Friedgut–Pilpel conjecture. For any n∈Nn\in\mathbb N and 1≤t≤n1\leq t\leq n, the maximum size of a tt-intersecting family in SnS_n is attained by one of the families Fn,t,r\mathcal F_{n,t,r}. In particular, for all t<n/2t<n/2, the maximum is attained by the tt-umvirate

Fn,t,0={σ∈Sn:Fixed(σ)⊃[t]}.\mathcal F_{n,t,0}=\{\sigma\in S_n:\mathrm{Fixed}(\sigma)\supset [t]\}.

Furthermore, every maximum-sized tt-intersecting family is a double translate τ1Fn,t,rτ2\tau_1\mathcal F_{n,t,r}\tau_2 for some rr and some τ1,τ2∈Sn\tau_1,\tau_2\in S_n. The source presents this as a daring conjecture after noting earlier partial results; the supplied status is unknown.

References

Primary source

Nathan Keller, Andrey Kupavskii, Noam Lifshitz and Ohad Sheinfeld, “A Complete Intersection Theorem for Large Permutation Groups”, arXiv:2607.00318 (2026).

Additional references

2 papers in this index state this conjecture (2024–2026). The statement above is taken from the most recent of them; the others are arXiv:2405.07843.

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