Using a global version of the equivariant Chern character, we describe the complexified twisted equivariant K-theory of a space with a compact Lie group action in terms of fixed-point data. We apply this to the case of a compact group acting on itself by conjugation and relate the result to the Verlinde algebra and to the Kac numerator at q = 1. Verlinde's formula is also discussed in this context.
IntroductionLet X be a locally compact topological space acted upon by a compact Lie group G. The equivariant K-theory K * G (X) was defined by Atiyah and Segal; some foundational papers are [3,30]. Twisted versions of K-theory, both equivariant and not equivariant, have recently attracted some attention. The equivariant twistings that we consider are classified up to isomorphism by an equivariant cohomology class(A twisting is a representative cocycle for such a class in some model of equivariant cohomology.) For torsion, non-equivariant twistings, the relevant K-theory was first introduced in [14]; subsequent treatments ([28] and, more recently, [1, 5, 8]) remove the torsion assumption. We recall for convenience the topologists' definition in the case ε = 0. Because the projective unitary group PU has classifying space K(Z; 3), a class [τ ] ∈ H 3 (X; Z) defines a principal PU-bundle over X up to isomorphism. Let F X be the associated bundle of Fredholm endomorphisms. The negative τ K(X) groups are the homotopy groups of the space of sections of F X ; the others are determined by Bott periodicity. In the presence of a group action, the equivariant τ K-groups arise from the space of invariant sections (a technical variation is needed in the equivariant case, pertaining to the topology on Fredholm operators [5, 31]). In this paper, we shall implicitly assume the basic topological properties of twisted K-theory; their justification is found in the papers mentioned earlier. However, since H 1 twistings get less coverage in the literature, we discuss them in more detail in § 4.One of the basic results [30] of the equivariant theory expresses, in terms of fixed-point data, the localisation of K * G (X) at prime ideals in the representation ring R G of G. The situation simplifies considerably after complexification, when the maximal ideals in R G correspond to (complex semi-simple) conjugacy classes. Recall that, in the non-equivariant case, the Chern character maps complex K-theory isomorphically onto complex cohomology. The localisation results can be assembled into a description of complex equivariant K-theory by a globalised Chern character [4,29,32] supported over the entire group. Part I of our paper generalises these results to the twisted case: the main result, Theorem 3.9, describes τ K * G (X), via the twisted Chern character ( § 2), in terms of (twisted) equivariant cohomology of fixed-point sets, with coefficients in certain equivariantly flat complex line bundles. For orbifolds, this is Vafa's discrete torsion [36,37]. We give a more detailed outline of Part I in § 1.In Part II, we apply our main result to the ...