Fusion Rings over Drinfeld Doubles

Abstract

The fusion rules in $\mathrm{Rep}_f D(G)$ for a finite group $G$ can be computed in terms of character inner products. Using an explicit formula for these fusion rules, we show that $\mathrm{Rep}_f D(G)$ is multiplicity free for two infinite families of finite groups: the Dihedral groups and the Dicyclic groups. In fact, we will compute all fusion rules in these categories. Multiplicity freeness is a desired property for modular tensor categories, since it greatly simplifies the computation of $F$-matrices. Furthermore, we observe that the fusion rules for Dihedral groups $D_{2n}$ with $n$ odd are extremely similar to the fusion rules of Type $B$ level $2$ fusion algebras of Wess-Zumino-Witten conformal field theories. Moreover, we give a proof of the fusion rule formula by using Mackey theory.