Ahlswede–Khachatrian-type conjecture for t-intersecting families of permutations

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Let nn and tt be positive integers, let cSigmancSigma_n be the collection of permutations of [n][n], and let cAkcA_k be the family

cAk={σ\incSigman:σ(i)=i for at least t+k indices i∈[t+2k]}.cA_k=\{\sigma\incSigma_n:\sigma(i)=i\text{ for at least }t+k\text{ indices }i\in[t+2k]\}.

A family \calF\subseteqcSigman\calF\subseteqcSigma_n is tt-intersecting if any two of its permutations agree on at least tt values of ii. Ahlswede–Khachatrian-type conjecture. If \calF\subseteqcSigman\calF\subseteqcSigma_n is a tt-intersecting family, then

∣\calF∣≤max⁡k∣\calAk∣.|\calF|\leq\max_k|\calA_k|.

Moreover, if equality holds, then \calF=σ\calAkτ\calF=\sigma\calA_k\tau for some kk and permutations σ\sigma and τ\tau. This is the permutation analogue of the Ahlswede–Khachatrian theorem for tt-intersecting families of sets. The conjecture asks both for the sharp maximum size and for a classification of all extremal families; its resolution is not specified in the supplied source.

References

Primary source

Pitchayut Saengrungkongka, “Extremal t-intersecting Families of Permutations for Large t”, arXiv:2605.26051 (2026).

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