Invariant K\"ahler potentials and symplectic reduction

Abstract

For a proper Hamiltonian action of a Lie group $G$ on a K\"ahler manifold $(X,\omega)$ with momentum map $\mu$ we show that the symplectic reduction $\mu^{-1}(0)/G$ is a normal complex space. Every point in $\mu^{-1}(0)$ has a $G$-stable open neighborhood on which $\omega$ and $\mu$ are given by a $G$-invariant K\"ahler potential. This is used to show that $\mu^{-1}(0)/G$ is a K\"ahler space. Furthermore we examine the existence of potentials away from $\mu^{-1}(0)$ with both positive and negative results.