Published online by Cambridge University Press: 09 June 2022
The automorphism group $\operatorname {Aut}(F_n)$ of the free group
$F_n$ acts on a space
$A_d(n)$ of Jacobi diagrams of degree d on n oriented arcs. We study the
$\operatorname {Aut}(F_n)$-module structure of
$A_d(n)$ by using two actions on the associated graded vector space of
$A_d(n)$: an action of the general linear group
$\operatorname {GL}(n,\mathbb {Z})$ and an action of the graded Lie algebra
$\mathrm {gr}(\operatorname {IA}(n))$ of the IA-automorphism group
$\operatorname {IA}(n)$ of
$F_n$ associated with its lower central series. We extend the action of
$\mathrm {gr}(\operatorname {IA}(n))$ to an action of the associated graded Lie algebra of the Andreadakis filtration of the endomorphism monoid of
$F_n$. By using this action, we study the
$\operatorname {Aut}(F_n)$-module structure of
$A_d(n)$. We obtain an indecomposable decomposition of
$A_d(n)$ as
$\operatorname {Aut}(F_n)$-modules for
$n\geq 2d$. Moreover, we obtain the radical filtration of
$A_d(n)$ for
$n\geq 2d$ and the socle of
$A_3(n)$.