English

2-intersecting permutations

Combinatorics 2021-09-07 v1

Abstract

In this paper we consider the Erd\H{o}s-Ko-Rado property for both 22-pointwise and 22-setwise intersecting permutations. Two permutations σ,τSym(n)\sigma,\tau \in Sym(n) are tt-setwise intersecting if there exists a tt-subset SS of {1,2,,n}\{1,2,\dots,n\} such that Sσ=SτS^\sigma = S^\tau. If for each sSs\in S, sσ=sτs^\sigma = s^\tau, then we say σ\sigma and τ\tau are tt-pointwise intersecting. We say that Sym(n)Sym(n) has the tt-setwise (resp. tt-pointwise) intersecting property if for any family F\mathcal{F} of tt-setwise (resp. tt-pointwise) intersecting permutations, F(nt)!t!|\mathcal{F}| \leq (n-t)!t! (resp. F(nt)!|\mathcal{F}| \leq (n-t)!). Ellis (["Setwise intersecting families of permutation". { Journal of Combinatorial Theory, Series A}, 119(4):825-849, 2012.]), proved that for nn sufficiently large relative to tt, Sym(n)Sym(n) has the tt-setwise intersecting property. Ellis also conjuctured that this result holds for all ntn \geq t. Ellis, Friedgut and Pilpel [Ellis, David, Ehud Friedgut, and Haran Pilpel. "Intersecting families of permutations." {Journal of the American Mathematical Society} 24(3):649-682, 2011.] also proved that for nn sufficiently large relative to tt, Sym(n)Sym(n) has the tt-pointwise intersecting property. It is also conjectured that Sym(n)Sym(n) has the tt-pointwise intersecting propoperty for n2t+1n\geq 2t+1. In this work, we prove these two conjectures for Sym(n)Sym(n) when t=2t=2.

Keywords

Cite

@article{arxiv.2005.00139,
  title  = {2-intersecting permutations},
  author = {Karen Meagher and A. S. Razafimahatratra},
  journal= {arXiv preprint arXiv:2005.00139},
  year   = {2021}
}

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18 pages