Hopf $2$-cocycles for certain affine algebraic reductive groups

Abstract

Let $G$ be an affine algebraic reductive group over $\mathbb{C}$ whose identity component is a torus $T$, and let $K:=G/T$. We realize $\Rep(G)$ as an equivariantization $\Rep(T)^K$ and describe its finite indecomposable semisimple module categories in terms of equivariant module-category data over $\Rep(T)$. For such an equivariantization, rank one is characterized by transitivity of the induced action on the simple objects of the underlying $\Rep(T)$-module category, and nondegeneracy of a projective cocycle on a point stabilizer. We use this criterion to parametrize fiber functors on $\Rep(G)$, prove that every such fiber functor is classical, hence arises from a Hopf $2$-cocycle on $\mathcal{O}(G)$, and classify (minimal) Hopf $2$-cocycle on $\mathcal{O}(G)$. For commutative direct products $G=T\times K$, we also give the canonical Künneth decomposition of the group of gauge classes, including the mixed component, and compare it with our classification.

23 pages