The cyclotomic double shuffle torsor in terms of Betti and de Rham coproducts

Abstract

To describe the double shuffle relations between multiple polylogarithm values at $N$th roots of unity, Racinet attached to each finite cyclic group $G$ of order $N$ and each group embedding $\iota : G \to \mathbb{C}^{\times}$, a $\mathbb{Q}$-scheme $\mathsf{DMR}^{\iota}$ which associates to each commutative $\mathbb{Q}$-algebra $\mathbf{k}$, a set $\mathsf{DMR}^{\iota}(\mathbf{k})$ that can be decomposed as a disjoint union of sets $\mathsf{DMR}^{\iota}_{\lambda}(\mathbf{k})$ with $\lambda \in \mathbf{k}$. He also exhibited a $\mathbb{Q}$-group scheme $\mathsf{DMR}_0^G$ and showed that $\mathsf{DMR}^{\iota}_{\lambda}(\mathbf{k})$ is a torsor for the action of $\mathsf{DMR}_0^G(\mathbf{k})$. Then, Enriquez and Furusho showed for $N=1$ that a subscheme $\mathsf{DMR}^{\iota}_{\times}$ of $\mathsf{DMR}^{\iota}$ is a torsor of isomorphisms relating de Rham and Betti objects. In previous work, we reformulated Racinet's construction in terms of crossed products and identified his coproduct with a coproduct $\widehat{\Delta}^{\mathcal{M}, \mathrm{DR}}_G$ defined on a module $\widehat{\mathcal{M}}_G^{\mathrm{DR}}$ over an algebra $\widehat{\mathcal{W}}_G^{\mathrm{DR}}$ equipped with its own coproduct $\widehat{\Delta}^{\mathcal{W}, \mathrm{DR}}_G$. In this paper, we provide a generalization of Enriquez and Furusho's result to any $N \geq 1$: we exhibit a module $\widehat{\mathcal{M}}_N^{\mathrm{B}}$ over an algebra $\widehat{\mathcal{W}}_N^{\mathrm{B}}$ and show the existence of compatible coproducts $\widehat{\Delta}^{\mathcal{W}, \mathrm{B}}_N$ and $\widehat{\Delta}^{\mathcal{M}, \mathrm{B}}_N$ such that $\mathsf{DMR}^{\iota}_{\times}$ is contained in the torsor of isomorphisms relating $\widehat{\Delta}^{\mathcal{W}, \mathrm{B}}_N$ (resp. $\widehat{\Delta}^{\mathcal{M}, \mathrm{B}}_N$) to $\widehat{\Delta}^{\mathcal{W}, \mathrm{DR}}_G$ (resp. $\widehat{\Delta}^{\mathcal{M}, \mathrm{DR}}_G$).