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    Forbidding just one intersection, for permutations

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    We prove that for nn sufficiently large, if AA is a family of permutations of {1,2,…,n}\{1,2,\ldots,n\} with no two permutations in A\mathcal{A} agreeing exactly once, then ∣Aβˆ£β‰€(nβˆ’2)!|\mathcal{A}| \leq (n-2)!, with equality holding only if A\mathcal{A} is a coset of the stabilizer of 2 points. We also obtain a Hilton-Milner type result, namely that if A\mathcal{A} is such a family which is not contained within a coset of the stabilizer of 2 points, then it is no larger than the family $\{\sigma \in S_{n}:\ \sigma(1)=1,\sigma(2)=2,\ \#\{\textrm{fixed points of}\sigma \geq 5\} \neq 1\} \cup \{(1\ 3)(2\ 4),(1\ 4)(2\ 3),(1\ 3\ 2\ 4),(1\ 4\ 2\ 3)\}.Weconjecturethatfor. We conjecture that for t \in \mathbb{N},andfor, and for nsufficientlylargedependingon sufficiently large depending on t,if, if \mathcal{A}isfamilyofpermutationsof is family of permutations of \{1,2,\ldots,n\}withnotwopermutationsin with no two permutations in \mathcal{A}agreeingexactly agreeing exactly t-1times,then times, then |\mathcal{A}| \leq (n-t)!,withequalityholdingonlyif, with equality holding only if \mathcal{A}isacosetofthestabilizerof is a coset of the stabilizer of tpoints.ThiscanbeseenasapermutationanalogueofaconjectureofErdo˝sonfamiliesof points. This can be seen as a permutation analogue of a conjecture of Erd\H{o}s on families of k$-element sets with a forbidden intersection, proved by Frankl and F\"uredi in [P. Frankl and Z. F\"uredi, Forbidding Just One Intersection, Journal of Combinatorial Theory, Series A, Volume 39 (1985), pp. 160-176].Comment: 26 page
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