Modular invariant Frobenius algebras from ribbon Hopf algebra automorphisms

For any finite-dimensional factorizable ribbon Hopf algebra H and any ribbon automorphism of H, we establish the existence of the following structure: an H-bimodule F-omega and a bimodule morphism Z(omega) from Lyubashenko's Hopf algebra object K for the bimodule category to F-omega. This morphism is invariant under the natural action of the mapping class group of the one-punctured torus on the space of bimodule morphisms from K to F-omega. We further show that the bimodule F-omega can be endowed with a natural structure of a commutative symmetric Frobenius algebra in the monoidal category of H-bimodules, and that it is a special Frobenius algebra iff H is semisimple. The bimodules K and F-omega can both be characterized as coends of suitable bifunctors. The morphism Z(omega) is obtained by applying a monodromy operation to the coproduct of F-omega; a similar construction for the product of F-omega exists as well. Our results are motivated by the quest to understand the bulk state space and the bulk partition function in two-dimensional conformal field theories with chiral algebras that are not necessarily semisimple. (C) 2012 Elsevier Inc. All rights reserved.