Simple Functors of Admissible Linear Categories

Generalizing an idea used by Bouc, Thevenaz, Webb and others, we introduce the notion of an admissible R-linear category for a commutative unital ring R. Given an R-linear category L, we define an L-functor to be a functor from L to the category of R-modules. In the case where L is admissible, we establish a bijective correspondence between the isomorphism classes of simple functors and the equivalence classes of pairs (G, V) where G is an object and V is a module of a certain quotient of the endomorphism algebra of G. Here, two pairs (F, U) and (G, V) are equivalent provided there exists an isomorphism F <- G effecting transport to U from V. We apply this to the category of finite abelian p-groups and to a class of subcategories of the biset category.