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Polite actions of non-compact Lie groups

Based mainly on examples of interest in mechanics, we define the notion of a polite group action. One may view this as not only trying to give a more general notion than properness of a group action, but also to more fully understand the role of invariant functions in describing just about everything of interest in reduction. We show that a polite action of a symmetry group of a dynamical system admits reduction and reconstruction.