English

No Hilton-Milner type results for linear groups of degree two

Combinatorics 2021-11-11 v2

Abstract

A set of permutations F\mathcal{F} of a finite transitive permutation group GSym(Ω)G\leq \operatorname{Sym}(\Omega) is \emph{intersecting} if any pair of elements of F\mathcal{F} agree on an element of Ω\Omega. We say that GG has the \emph{EKR property} if an intersecting set of GG has size at most the order of a point stabilizer. Moreover, GG has the \emph{strict-EKR} property whenever GG has the EKR property and any intersecting set of maximum size is a coset of a point stabilizer of GG. It is known that the permutation group GL2(Fq)\operatorname{GL}_2(\mathbb{F}_q) acting on Ωq:=Fq2{0}\Omega_q := \mathbb{F}_q^2\setminus\{0\} has the EKR property, but does not have the strict-EKR property since the stabilizer of a hyperplane is a maximum intersecting set. In this paper, it is proved that the Hilton-Milner type result does not hold for GL2(Fq)\operatorname{GL}_2(\mathbb{F}_q) acting on Ωq\Omega_q. Precisely, it is shown that a maximal intersecting set of GL2(Fq)\operatorname{GL}_2(\mathbb{F}_q) is of maximum size. As a result, we prove the Complete Erd\H{o}s-Ko-Rado theorem for GL2(Fq)\operatorname{GL}_2(\mathbb{F}_{q}).

Keywords

Cite

@article{arxiv.2111.03829,
  title  = {No Hilton-Milner type results for linear groups of degree two},
  author = {Roghayeh Maleki and Andriaherimanana Sarobidy Razafimahatratra},
  journal= {arXiv preprint arXiv:2111.03829},
  year   = {2021}
}

Comments

10 pages. We extended the EKR property to any transitive subgroup