Bigraded Lie algebras related to MZVs
We prove that Goncharov's dihedral Lie coalgebra $D_{\bullet\bullet}:={\oplus}_{k\geq m \geq 1} D_{m,k}$ of the trivial group ($\widehat{\mathscr{D}}_{\bullet \bullet}(G)$ of (arxiv:math/0009121) for $G=\{e\}$) is the bigraded dual of Brown's linearized double shuffle Lie algebra $\mathfrak{ls}:={\oplus}_{k\geq m \geq 1}\mathfrak{ls}_m^k\subset \mathbb{Q}\langle x,z \rangle$ whose Lie bracket is the Ihara bracket initially defined over $\mathbb{Q}\langle x,z \rangle$. This by constructing an explicit isomorphism of bigraded Lie coalgebras $D_{\bullet \bullet} \to \mathfrak{ls}^\vee$, where $\mathfrak{ls}^\vee$ is the Lie coalgebra dual in the bigraded sense to $\mathfrak{ls}$. The work leads to the equivalence between the two statements: "$D_{\bullet \bullet}$ is a Lie coalgebra with respect to Goncharov's cobracket formula" and "$ \mathfrak{ls}$ is preserved by the Ihara bracket". We also prove folklore results (that apparently have no written proofs in the literature) stating that for $m \geq 2$: $D_{m,\bullet}:=\oplus_{k\geq m} D_{m,k}$ is graded isomorphic (dual) to Ihara-Kaneko-Zagier's double shuffle space $\mathrm{Dsh}_{m}:=\oplus_{k\geq m} \mathrm{Dsh}_{m}({{k}-m}) \subset \mathbb{Q}[x_1,\dots,x_m]$, and that a given linear map $f_m: \mathbb{Q}\langle x,z \rangle_m \to \mathbb{Q}[x_1,\dots,x_m]$, where $\mathbb{Q}\langle x,z \rangle_m$ is the space linearly generated by monomials of $\mathbb{Q}\langle x,z \rangle$ of degree $m$ with respect to $z$, restricts to a graded isomorphism $\bar{f}_m: \mathfrak{ls}_m:=\oplus_{k\geq m} \mathfrak{ls}_m^k \to \mathrm{Dsh}_{m}$. Here, we establish three explicit compatible isomorphisms $D_{\bullet \bullet} \to \mathfrak{ls}^\vee, D_{m\bullet}\to \mathrm{Dsh}_{m}^\vee$ and $\bar{f}_m: \mathfrak{ls}_m \to \mathrm{Dsh}_{m}$, where $\mathrm{Dsh}_{m}^\vee$ is the graded dual of $\mathrm{Dsh}_{m}$.