Algebraic properties of the group of germs of diffeomorphisms

Abstract

We establish some algebraic properties of the group $\mathrm{Diff}({\mathbb{C}}^n,0)$ of germs of analytic diffeomorphisms of ${\mathbb{C}}^n$, and its formal completion $\widehat{\mathrm{Diff}}({\mathbb{C}}^n,0)$. For instance we describe the commutator of $\mathrm{Diff}({\mathbb{C}}^n,0)$, but also prove that any finitely generated subgroup of $\mathrm{Diff}({\mathbb{C}}^n,0)$ is residually finite; we thus obtain some constraints of groups that embed into $\mathrm{Diff}({\mathbb{C}}^n,0)$. We show that $\widehat{\mathrm{Diff}}({\mathbb{C}}^n,0)$ is an Hopfian group, and that $\mathrm{Diff}({\mathbb{C}}^n,0)$ and $\widehat{\mathrm{Diff}}({\mathbb{C}}^n,0)$ are not co-Hopfian. We end by the description of the automorphisms groups of $\widehat{\mathrm{Diff}}(\mathbb{C},0)$, and $\mathrm{Diff}(\mathbb{C},0)$.