Matrix Nonlinear Schrödinger Equations and Moment Maps into Loop Algebras${}^†$

Abstract

It is shown how Darboux coordinates on a reduced symplectic vector space may be used to parametrize the phase space on which the finite gap solutions of matrix nonlinear Schrödinger equations are realized as isospectral Hamiltonian flows. The parametrization follows from a moment map embedding of the symplectic vector space, reduced by suitable group actions, into the dual $\tilde\grg^{+*}$ of the algebra $\tilde\grg^+$ of positive frequency loops in a Lie algebra $\grg$. The resulting phase space is identified with a Poisson subspace of $\tilde\grg^{+*}$ consisting of elements that are rational in the loop parameter. Reduced coordinates associated to the various Hermitian symmetric Lie algebras $(\grg,\grk)$ corresponding to the classical Lie algebras are obtained.