Invariants from the Sweedler power maps on integrals

Abstract

For a finite-dimensional Hopf algebra $A$ with a nonzero left integral $\Lambda$, we investigate a relationship between $P_n(\Lambda )$ and $P_n^J(\Lambda )$, where $P_n$ and $P_n^J$ are respectively the $n$-th Sweedler power maps of $A$ and the twisted Hopf algebra $A^J$. We use this relation to give several invariants of the representation category Rep$(A)$ considered as a tensor category. As applications, we distinguish the representation categories of 12-dimensional pointed nonsemisimple Hopf algebras. Also, these invariants are sufficient to distinguish the representation categories Rep$(K_8)$, Rep$(\kk Q_8)$ and Rep$(\kk D_4)$, although they have been completely distinguished by their Frobenius-Schur indicators. We further reveal a relationship between the right integrals $\lambda$ in $A^{\ast }$ and ${\lambda }^J$ in $(A^J{)}^{\ast }$. This can be used to give a uniform proof of the remarkable result which says that the $n$-th indicator ${\nu }_n(A)$ is a gauge invariant of $A$ for any $n\in \mathbb{Z}$. We also use the expression for ${\lambda }^J$ to give an alternative proof of the known result that the Killing form of the Hopf algebra $A$ is invariant under twisting. As a result, the dimension of the Killing radical of $A$ is a gauge invariant of $A$.

19 pages,comments are welcome