Nil-automorphisms of groups with residual properties

TL;DR

This paper studies nil-automorphisms and unipotent automorphisms in groups with residual properties, proving that finite groups of nil-automorphisms are nilpotent and that unipotent automorphism groups are locally-nilpotent, offering new insights into Engel-related automorphisms.

Contribution

It extends the understanding of automorphisms with residual properties, establishing new nilpotency and local-nilpotency results for these automorphism groups.

Findings

Finite nil-automorphism groups of locally graded groups are nilpotent.

Groups of unipotent automorphisms in residually-finite and profinite groups are locally-nilpotent.

Provides an alternative proof of Wilson's theorem on n-Engel residually-finite groups.

Abstract

Following Plotkin we say that the automorphism $x$ of the group $G$ is a nil-automorphism if, for every $g\in G$, there exists $n=n(g)$ such that $[g,_n x]=1$. If the integer $n$ can be chosen independently of $g$, then $x$ is said to be unipotent. Nil and unipotent automorphisms can be regarded as a natural extension of the concept of Engel element, since a nil-automorphism $x$ is just a left-Engel element in $G < x >$. In this paper we consider nil-automorphisms of groups with residual properties namely locally-graded groups, residually-finite groups and profinite groups. The first result we prove says that a finite group of nil-automorphisms of a locally graded group, must be nilpotent. Next we turn our attention to groups of unipotent automorphisms of residually-finite and profinite groups. We show that such groups are locally-nilpotent and, as a by-product, we obtain an alternative proof of a well known theorem of Wilson about n-Engel residually-finite groups.