Taylor expansions of groups and filtered-formality

Abstract

Let $G$ be a finitely generated group, and let $\mathbb{k}G$ be its group algebra over a field of characteristic $0$. A Taylor expansion is a certain type of map from $G$ to the degree completion of the associated graded algebra of $\mathbb{k}G$ which generalizes the Magnus expansion of a free group. The group $G$ is said to be filtered-formal if its Malcev Lie algebra is isomorphic to the degree completion of its associated graded Lie algebra. We show that $G$ is filtered-formal if and only if it admits a Taylor expansion, and derive some consequences.

19 pages; accepted for publication in the European Journal of Mathematics