Surjectivity for Hamiltonian loop group spaces (Q1879012)

Let \(G\) be a compact Lie group, and let \(LG\) denote the corresponding loop group. Let \((X,\omega)\) be a weakly symplectic Banach manifold. The authors study a Hamiltonian action of \(LG\) on \((X,\omega)\) with the proper moment map \(\mu:M\rightarrow Lg^{\ast}\). They consider the function \(| \mu| ^2\), and use a version of Morse theory to show that the inclusion map \(j:\mu^{-1}(0)\rightarrow X \) induces a surjection \(j^{\ast}:H_G^{\ast}(X)\rightarrow H_G^{\ast}(\mu^{-1}(0))\), in analogy with Kirwan's surjectivity theorem in the finite-dimensional case. They also prove a version of this surjectivity theorem for quasi-Hamiltonian \(G\)-spaces.