English

Forbidding just one intersection, for permutations

Combinatorics 2013-10-31 v1

Abstract

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 A(n2)!|\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 {σSn: σ(1)=1,σ(2)=2, #{fixed points ofσ5}1}{(1 3)(2 4),(1 4)(2 3),(1 3 2 4),(1 4 2 3)}\{\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)\}. We conjecture that for tNt \in \mathbb{N}, and for nn sufficiently large depending on tt, if A\mathcal{A} is family of permutations of {1,2,,n}\{1,2,\ldots,n\} with no two permutations in A\mathcal{A} agreeing exactly t1t-1 times, then A(nt)!|\mathcal{A}| \leq (n-t)!, with equality holding only if A\mathcal{A} is a coset of the stabilizer of tt points. This can be seen as a permutation analogue of a conjecture of Erd\H{o}s on families of kk-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].

Keywords

Cite

@article{arxiv.1310.8108,
  title  = {Forbidding just one intersection, for permutations},
  author = {David Ellis},
  journal= {arXiv preprint arXiv:1310.8108},
  year   = {2013}
}

Comments

26 pages

R2 v1 2026-06-22T01:57:18.841Z