Enveloping actions and Takai duality for partial actions

We show that any partial action on a topological space X is the restriction of a suitable global action, called enveloping action, that is essentially unique. In the case of C*-algebras, we prove that any partial action has a unique enveloping action up to Morita equivalence, and that the corresponding reduced crossed products are Morita equivalent. The study of the enveloping action up to Morita equivalence reveals the form that Takai duality takes for partial actions. By applying our constructions, we prove that the reduced crossed product of the reduced cross-sectional algebra of a Fell bundle by the dual coaction is liminal, postliminal, or nuclear, if and only if so is the unit fiber of the bundle. We also give a non-commutative generalization of the well-known fact that the integral curves of a vector field on a compact manifold are defined on all of R. (C) 2002 Elsevier Science (USA). All rights reserved.