Abstract. Let G be a reductive group, defined over the Galois field Fp with p being good for G. Using support varieties and covering techniques based on GrT -modules, we determine the position of simple modules and baby Verma modules within the stable Auslander-Reiten quiver Γs(Gr) of the r-th Frobenius kernel of G. In particular, we show that the almost split sequences terminating in these modules usually have an indecomposable middle term.Concerning support varieties, we introduce a reduction technique leading to isomorphismsfor baby Verma modules of certain highest weights λ, µ ∈ X(T ), which are related by the notion of depth.
IntroductionIn the representation theory of finite-dimensional self-injective algebras, the stable AuslanderReiten quiver has proven to be an important homological invariant, which has been studied for group algebras of finite groups, reduced enveloping algebras of restricted Lie algebras and distribution algebras of infinitesimal group schemes. In the classical context of Frobenius kernels of reductive groups, the relevant algebras are usually of wild representation type, rendering a classification of their indecomposable modules a hopeless task. This fact notwithstanding, one does have a fairly good understanding of the connected components of the corresponding stable Auslander-Reiten quiver. It is therefore of interest to relate this information to certain classes of indecomposable modules, such as simple modules, Weyl modules or Verma modules, and to identify the position of the latter within the AR-quiver. The main problem in this undertaking is the lack of a suitable presentation of the underlying algebras, as required by the techniques of abstract representation theory. On the other hand, the theory of rank varieties and support varieties has seen considerable progress over the last years, so that one can hope to exploit these tools in the aforementioned context.In continuation of work begun in [22], we study in this article those Auslander-Reiten components of the algebras Dist(G r ) that contain simple modules or baby Verma modules. The underlying algebra Dist(G r ) consists of the distributions of the r-th Frobenius kernel of the smooth reductive group scheme G, defined over an algebraically closed field of positive characteristic p. The case r = 1, which pertains to restricted enveloping algebras of reductive Lie algebras, was settled in [22] by means of a detailed analysis of nilpotent orbits in rank varieties, leading to the consideration of groups of types SL(2) 1 ×SL(2) 1 , SL(3) 1 and SO(5) 1 . For r > 1, rank varieties are less tractable and the structure of the cohomology rings defining support varieties is more complicated. We address these problems by passage to G r T -modules, as described below. With regard to Auslander-Reiten theory, our main results can roughly be summarized as follows:Theorem. Let G be a smooth reductive group scheme, defined over F p . Given a character λ ∈ X(T ) and r ≥ 1, the following statements hold: