Galois actions of finitely generated groups rarely have model companions

Abstract

We show that if $G$ is a finitely generated group such that its profinite completion $\hat{G}$ is ``far from being projective'' (that is the kernel of the universal Frattini cover of $\hat{G}$ 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.

Few typos corrected, added the part about free products