Chern character for twisted K-theory of orbifolds

For an orbifold X and alpha is an element of H-3 (X, Z), we introduce the twisted cohomology H-c* (X, alpha) and prove that the non-commutative Chem character of Connes-Karoubi establishes an isomorphism between the twisted K-groups K-alpha*(X) circle times C and the twisted cohomology H-c*(X, alpha). This theorem, on the one hand, generalizes a classical result of Baum-Connes, Brylinski-Nistor, and others, that if X is an orbifold then the Chem character establishes an isomorphism between the K-groups of X tensored with C, and the compactly-supported cohomology of the inertia orbifold. On the other hand, it also generalizes a recent result of Adem-Ruan regarding the Chem character isomorphism of twisted orbifold K-theory when the orbifold is a global quotient by a finite group and the twist is a special torsion class, as well as Mathai-Stevenson's theorem regarding the Chem character isomorphism of twisted K-theory of a compact manifold. (C) 2005 Elseviei Inc. All rights reserved.