English

$3$-setwise intersecting families of the symmetric group

Combinatorics 2021-09-07 v2

Abstract

Given two positive integers n3n\geq 3 and tnt\leq n, the permutations σ,πSym(n)\sigma,\pi \in \operatorname{Sym}(n) are tt-setwise intersecting if they agree (setwise) on a tt-subset of {1,2,,n}\{1,2,\ldots,n\}. A family FSym(n)\mathcal{F} \subset \operatorname{Sym}(n) is tt-setwise intersecting if any two permutations of F\mathcal{F} are tt-setwise intersecting. Ellis [Journal of Combinatorial Theory, Series A, 119(4), 825--849, 2012] conjectured that if tnt\leq n and FSym(n)\mathcal{F} \subset \operatorname{Sym}(n) is a tt-setwise intersecting family, then Ft!(nt)!|\mathcal{F}|\leq t!(n-t)! and equality holds only if F\mathcal{F} is a coset of a setwise stablizer of a tt-subset of {1,2,,n}\{1,2,\ldots,n\}. In this paper, we prove that if n11n\geq 11 and F\mathcal{F} is 33-setwise intersecting, then F6(n3)!|\mathcal{F}|\leq 6(n-3)!. Moreover, we prove that the characteristic vector of a 33-setwise intersecting family of maximum size lies in the sum of the eigenspaces induced by the permutation module of Sym(n)\operatorname{Sym}(n) acting on the 33-subsets of {1,2,,n}\{1,2,\ldots,n\}.

Keywords

Cite

@article{arxiv.2010.00229,
  title  = {$3$-setwise intersecting families of the symmetric group},
  author = {Angelot Behajaina and Roghayeh Maleki and Aina Toky Rasoamanana and A. Sarobidy Razafimahatratra},
  journal= {arXiv preprint arXiv:2010.00229},
  year   = {2021}
}

Comments

19 pages, a link to our code was added, typos and grammar mistakes were fixed