Twisted Orbifold K-Theory

We use equivariant methods to establish basic properties of orbifold K-theory. We introduce the notion of twisted orbifold K-theory in the presence of discrete torsion, and show how it can be explicitly computed for global quotients.Comment: 30 page

On some examples of group actions and group extensions

We study and relate certain actions and extensions involving 2-groups

On the structure of spaces of commuting elements in compact Lie groups

In this note we study topological invariants of the spaces of homomorphisms Hom(\pi,G), where \pi\ is a finitely generated abelian group and G is a compact Lie group arising as an arbitrary finite product of the classical groups SU(r), U(q) and Sp(k).Comment: Accepted for publication in the proceedings of the conference: Configuration spaces: Geometry, Combinatorics and Topology. Scuola Normale Superior