Crossed product interpretation of the Double Shuffle Lie algebra attached to a finite Abelian group

Abstract

Racinet studied the scheme associated with the double shuffle and regularization relations between multiple polylogarithm values at $N^{th}$ roots of unity and constructed a group scheme attached to the situation; he also showed it to be the specialization for $G=\mu_N$ of a group scheme $\mathsf{DMR}_0^G$ attached to a finite abelian group $G$. Then, Enriquez and Furusho proved that $\mathsf{DMR}_0^G$ can be essentially identified with the stabilizer of a coproduct element arising in Racinet's theory with respect to the action of a group of automorphisms of a free Lie algebra attached to $G$. We reformulate Racinet's construction in terms of crossed products. Racinet's coproduct can then be identified with a coproduct $\hat{\Delta}^{\mathcal{M}}_G$ defined on a module $\hat{\mathcal{M}}_G$ over an algebra $\hat{\mathcal{W}}_G$, which is equipped with its own coproduct $\hat{\Delta}^{\mathcal{W}}_G$, and the group action on $\hat{\mathcal{M}}_G$ extends to a compatible action of $\hat{\mathcal{W}}_G$. We then show that the stabilizer of $\hat{\Delta}^{\mathcal{M}}_G$, hence $\mathsf{DMR}_0^G$, is contained in the stabilizer of $\hat{\Delta}^{\mathcal{W}}_G$. This yields an explicit group scheme containing $\mathsf{DMR}_0^G$, which we also express in the Racinet formalism.