On the stable Andreadakis problem

Abstract

Let $F\_n$ be the free group on $n$ generators. Consider the group $IA\_n$ of automorpisms of $F\_n$ acting trivially on its abelianization. There are two canonical filtrations on $IA\_n$: the first one is its lower central series $Γ\_*$; the second one is the Andreadakis filtration $\mathcal A\_*$, defined from the action on $F\_n$. In this paper, we establish that the canonical morphism between the associated graded Lie rings ${\mathcal L}(Γ\_*)$ and ${\mathcal L}(\mathcal A\_*)$ is stably surjective. We then investigate a $p$-restricted version of the Andreadakis problem. A calculation of the Lie algebra of the classical congruence group is also included.