Topological Manin pairs and $(n,s)$-type series

Abstract

Lie subalgebras of $ L = \mathfrak{g}(\!(x)\!) \times \mathfrak{g}[x]/x^n\mathfrak{g}[x] $, complementary to the diagonal embedding $\Delta$ of $ \mathfrak{g}[\![x]\!] $ and Lagrangian with respect to some particular form, are in bijection with formal classical $r$-matrices and topological Lie bialgebra structures on the Lie algebra of formal power series $ \mathfrak{g}[\![x]\!] $. In this work we consider arbitrary subspaces of $ L $ complementary to $\Delta$ and associate them with so-called series of type $ (n,s) $. We prove that Lagrangian subspaces are in bijection with skew-symmetric $ (n,s) $-type series and topological quasi-Lie bialgebra structures on $ \mathfrak{g}[\![x]\!] $. Using the classificaiton of Manin pairs we classify up to twisting and coordinate transformations all quasi-Lie bialgebra structures. Series of type $ (n,s) $, solving the generalized Yang-Baxter equation, correspond to subalgebras of $L$. We discuss their possible utility in the theory of integrable systems.