Galois actions of finitely generated groups rarely have model companions

TL;DR

This paper demonstrates that for certain finitely generated groups with complex profinite completions, the class of existentially closed group actions on fields cannot be characterized by a first-order theory, correcting a previous error.

Contribution

It establishes that finitely generated groups with complex profinite completions do not have model companions for their actions on fields, correcting earlier results.

Findings

Existentially closed G-actions on fields are not elementary for certain groups.

Finitely generated, virtually free, non-free groups are 'far from being projective'.

The paper corrects a previous theorem about model theory of fields with group actions.

Abstract

We show that if $G$ is a finitely generated group such that its profinite completion $\widehat{G}$ is ``far from being projective'' (that is the kernel of the universal Frattini cover of $\widehat{G}$ is not a small profinite group), then the class of existentially closed $G$-actions on fields is not elementary. Since any infinite, finitely generated, virtually free, and not free group is ``far from being projective'', the main result of this paper corrects an error in our paper ``Model theory of fields with virtually free group actions'', Proc. London Math. Soc., (2) 118 (2019), 221--256 by showing the negation of Theorem 3.26 in that paper.