Structure of connected nested automorphism groups

Alexander Perepechko

Published 2023-12-13Version 1

Let $X$ be an affine algebraic variety and $\mathrm{Aut}(X)$ be its automorphism group. A subgroup $G\subset\mathrm{Aut}(X)$ is called nested if it is a countable increasing union of algebraic subgroups. We introduce so called dJ-like unipotent subgroups described by tuples of locally nilpotent derivations. We conjecture that any nested unipotent subgroup is contained in a dJ-like one. As an application, we show that under this conjecture any connected nested subgroup in $\mathrm{Aut}(X)$ is closed with respect to the ind-topology. Finally, we present a short proof that the de Jonqui\`eres subgroup of $\mathrm{Aut}(\mathbb{A}^n)$ is closed. We introduce the notion of a degree-preserving subgroup for that.