Topological Hochschild homology of twisted group algebras

Abstract

Let G be a group and A be a ring. There is a stable equivalence of orthogonal spectra ... between the topological Hochschild homology of the group algebra A[G] and the smash product of the topological Hochschild homology of A and the cyclic bar construction of G. This thesis generalizes this result to a twisted group algebra AT[G]. As an A-module, Ar[G] = A[G], but the multiplication is given by ag. a'g' = ag(a') gg', where G acts on A from the left through ring automorphisms. The main result is given in terms of a variant THH9(A) of the topological Hochschild spectrum that is equipped with a twisted cyclic structure inherited from the cyclic structure of the cyclic pointed space THH(A)[-]. We first define a parametrized orthogonal spectrum E(A, G) over the cyclic bar construction NCY(G). We prove there is a stable equivalence of spectra between the associated Thom spectrum of E(A, G) and THH(AT[G]). We then prove there is a stable equivalence of orthogonal spectra ... where the wedge-sum on the left hand side ranges over the conjugacy classes of elements of G and the equivalence depends on a choice of representative g E (g) of every conjugacy class of elements in G.