Relative Serre functor for comodule algebras

Let $\mathcal{C}$ be a finite tensor category, and let $\mathcal{M}$ be an exact left $\mathcal{C}$-module category. A relative Serre functor of $\mathcal{M}$, introduced by Fuchs, Schaumann and Schweigert, is an endofunctor $\mathbb{S}$ on $\mathcal{M}$ together with a natural isomorphism $\underline{\mathrm{Hom}}(M, N)^* \cong \underline{\mathrm{Hom}}(N, \mathbb{S}(M))$ for $M, N \in \mathcal{M}$, where $\underline{\mathrm{Hom}}$ is the internal Hom functor of $\mathcal{M}$. In this paper, we discuss the case where $\mathcal{C} = {}_H \mathfrak{M}$ and $\mathcal{M} = {}_L \mathfrak{M}$ for a finite-dimensional Hopf algebra $H$ and a finite-dimensional exact left $H$-comodule algebra $L$. We give an explicit description of a relative Serre functor of ${}_L \mathfrak{M}$ and its twisted module structure in terms of integrals of $H$ and the Frobenius structure of $L$. We also study pivotal structures on ${}_L \mathfrak{M}$ and give some explicit examples.Comment: 48 page

Frobenius-Schur indicators in Tambara-Yamagami categories

We introduce formulae of Frobenius-Schur indicators of simple objects of Tambara-Yamagami categories. By using techniques of the Fourier transform on finite abelian groups, we study some arithmetic properties of indicators.Comment: 21 page

Monoidal Morita invariants for finite group algebras

Two Hopf algebras are called monoidally Morita equivalent if module categories over them are equivalent as linear monoidal categories. We introduce monoidal Morita invariants for finite-dimensional Hopf algebras based on certain braid group representations arising from the Drinfeld double construction. As an application, we show, for any integer $n$, the number of elements of order $n$ is a monoidal Morita invariant for finite group algebras. We also describe relations between our construction and invariants of closed 3-manifolds due to Reshetikhin and Turaev.Comment: 25 pages; To appear in J. of Algebra. Main modifications are the following: (i) Verbose parts of the paper were summarized. (ii) Theorem 6.3 is added. (iii) The relation between Theorem 1.1 and works of Ng and Schauenburg is adde