Coprime commutators in profinite groups

Abstract

By a coprime commutator in a profinite group $G$ we mean any element of the form $[x,y]$, where $x,y\in G$ and $(|x|,|y|)=1$. It is well-known that the subgroup generated by the coprime commutators of $G$ is precisely the pronilpotent residual ${\hat{I}}_{\infty }^3(G)$. There are several recent works showing that finiteness conditions on the set of coprime commutators have strong impact on the properties of ${\hat{I}}_{\infty }^3(G)$ and, more generally, on the structure of $G$. In this paper we show that if the set of coprime commutators of a profinite group $G$ is covered by countably many procyclic subgroups, then ${\hat{I}}_{\infty }^3(G)$ is finite-by-procyclic. In particular, it follows that $G$ is finite-by-pronilpotent-by-abelian.

24 pages, revised version, to appear in Revista Matemática Complutense