English

When is the search of relatively maximal subgroups reduced to quotients?

Group Theory 2023-01-02 v1

Abstract

Let X{\mathfrak{X}} be a class of finite groups closed under taking subgroups, homomorphic images, and extensions. Denote by kX(G){\mathrm{k}}_{\mathfrak{X}}(G) the number of conjugacy classes X{\mathfrak{X}}-maximal subgroups of a finite group GG. The natural problem to describe up to conjugacy X{\mathfrak{X}}-maximal subgroups of a given finite group is complicated by the fact that it is not inductive. In particular, generally speaking, the image of an X{\mathfrak{X}}-maximal subgroup is not X{\mathfrak{X}}-maximal in the image of a homomorphism. Nevertheless, there are group homomorphisms which preserve the number of conjugacy classes of X{\mathfrak{X}}-maximal subgroups (for example, the homomorphisms whose kernels are X{\mathfrak{X}}-groups). Under such homomorphisms, the image of an X{\mathfrak{X}}-maximal subgroup is always X{\mathfrak{X}}-maximal and, moreover, there is a natural bijection between the conjugacy classes of X{\mathfrak{X}}-maximal subgroups of the image and preimage. All such homomorphisms are completely described in the paper. More precisely, it is proved that, for a homomorphism ϕ\phi from a group GG, the equality kX(G)=kX(imϕ){\mathrm{k}}_{\mathfrak{X}}(G)={\mathrm{k}}_{\mathfrak{X}}(\mathrm{im}\, \phi) holds if and only if kX(kerϕ)=1{\mathrm{k}}_{\mathfrak{X}}(\ker \phi)=1, which in turn is equivalent to the fact that the composition factors of the kernel of ϕ\phi belong to an explicitly given list.

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Cite

@article{arxiv.2110.15638,
  title  = {When is the search of relatively maximal subgroups reduced to quotients?},
  author = {Wenbin Guo and Danila O. Revin},
  journal= {arXiv preprint arXiv:2110.15638},
  year   = {2023}
}

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