On Radically Graded Finite-Dimensional Quasi-Hopf Algebras

Abstract

Let p be a prime, and denote the class of radically graded finite dimensional quasi-Hopf algebras over C, whose radical has codimension p, by RG(p). The purpose of this paper is to continue the structure theory of finite dimensional quasi-Hopf algebras started in math.QA/0310253 (p=2) and math.QA/0402159 (p>2). More specifically, we completely describe the class RG(p) for p>2. Namely, we show that if H\in RG(p) has a nontrivial associator, then the rank of H[1] over H[0] is \le 1. This yields the following classification of H\in RG(p), p>2, up to twist equivalence: (a) Duals of pointed Hopf algebras with p grouplike elements, classified in math.QA/9806074. (b) Group algebra of Z_p with associator defined by a 3-cocycle. (c) The algebras A(q), introduced in math.QA/0402159. This result implies, in particular, that if p>2 is a prime then any finite tensor category over C with exactly p simple objects which are all invertible must have Frobenius-Perron dimension p^N, N=1,2,3,4,5 or 7. In the second half of the paper we construct new examples of finite dimensional quasi-Hopf algebras H, which are not twist equivalent to a Hopf algebra. They are radically graded, and H/Rad(H)=C[Z_n^m], with a nontrivial associator. For instance, to every finite dimensional simple Lie algebra g and an odd integer n, coprime to 3 if g=G_2, we attach a quasi-Hopf algebra of dimension n^{dim(g)}.