English

Profinite groups with complemented closed subgroups

Group Theory 2025-07-29 v1

Abstract

A group GG is said to be a CC-group if every subgroup HH has a permutable complement, i.e. if there exists a subgroup KK of GG such that G=HKG=HK and HK=1H \cap K=1. In this paper, we study the profinite counterpart of this concept. We say that a profinite group GG is profinite-CC if every closed subgroup admits a closed permutable complement. We first give some equivalent variants of this condition and then we determine the structure of profinite-CC groups: they are the semidirect products G=BAG=B\ltimes A of two closed subgroups A=CriIaiA=\text{Cr}_{i\in I} \, \langle a_i \rangle and B=CrjJbjB=\text{Cr}_{j\in J} \, \langle b_j \rangle that are cartesian products of cyclic groups of prime order, and with every ai\langle a_i \rangle normal in GG. Finally, we show that a profinite-CC group is a CC-group if and only if it is torsion and G:Z(G)G<|G:Z(G)\overline{G'}|<\infty.

Keywords

Cite

@article{arxiv.2507.20791,
  title  = {Profinite groups with complemented closed subgroups},
  author = {Gustavo A. Fernández-Alcober and Giulia Sabatino},
  journal= {arXiv preprint arXiv:2507.20791},
  year   = {2025}
}

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10 pages