Reduction principles for proper actions

Abstract

Let $G$ be a Lie group acting properly on a smooth manifold $M$. If $M/G$ is connected, then we exhibit some simple and basic constructions for proper actions. In particular, we prove that the reduction principle in compact transformation groups holds for proper actions. As an application, we prove that a reduction principle holds for polar actions and for the integral invariant for isometric actions of Lie groups, called copolarity, which measures how far from being polar the action is. We also investigate symplectic actions. Hence we assume that $(M,\omega)$ is a symplectic manifold and the $G$ action on $M$ preserves $\omega$. %If $G$ is Abelian, we generalize results proved in \cite{Ben,DP1,DP2} The main result is the Equivalence Theorem for coisotropic actions, generalizing \cite[Theorem 3 p.267]{HW}. Finally, we completely characterize asystatic actions generalizing results proved in \cite{pg}.