Two-sided (two-cosided) Hopf modules and Doi-Hopf modules for quasi-Hopf algebras

Let $H$ be a finite dimensional quasi-Hopf algebra over a field $k$ and ${\mathfrak A}$ a right $H$-comodule algebra in the sense of Hausser and Nill. We first show that on the $k$-vector space {\mathfrak A}\ot H^* we can define an algebra structure, denoted by {\mathfrak A}\ovsm H^*, in the monoidal category of left $H$-modules (i.e. {\mathfrak A}\ovsm H^* is an $H$-module algebra. Then we will prove that the category of two-sided $({\mathfrak A}, H)$-bimodules \hba is isomorphic to the category of relative ({\mathfrak A}\ovsm H^*, H^*)-Hopf modules, as introduced in by Hausser and Nill. In the particular case where ${\mathfrak A}=H$, we will obtain a result announced by Nill. We will also introduce the categories of Doi-Hopf modules and two-sided two-cosided Hopf modules and we will show that they are in certain situations isomorphic to module categories.Comment: 31 page

Integrals for (dual) quasi-Hopf algebras. Applications

A classical result in the theory of Hopf algebras concerns the uniqueness and existence of integrals: for an arbitrary Hopf algebra, the integral space has dimension $\leq 1$, and for a finite dimensional Hopf algebra, this dimension is exaclty one. We generalize these results to quasi-Hopf algebras and dual quasi-Hopf algebras. In particular, it will follow that the bijectivity of the antipode follows from the other axioms of a finite dimensional quasi-Hopf algebra. We give a new version of the Fundamental Theorem for quasi-Hopf algebras. We show that a dual quasi-Hopf algebra is co-Frobenius if and only if it has a non-zero integral. In this case, the space of left or right integrals has dimension one.Comment: 25 pages; new version with minor correction

Hopf-Galois extensions and an exact sequence for HH-Picard groups

Let $H$ be a Hopf algebra, and $A$ an $H$-Galois extension. We investigate $H$-Morita autoequivalences of $A$, introduce the concept of $H$-Picard group, and we establish an exact sequence linking the $H$-Picard group of $A$ and the Picard group of $A^{{\rm co}H}$.Comment: 35 pages; to appear in J. Algebr