On Ext-transfer for Algebraic Groups

This paper builds upon the work of Cline [C] and Donkin [D1] to describe explicit equivalences between some categories associated to the category of rational modules for a reductive group G and categories associated to the category of rational modules for a Levi subgroup H . As an application, we establish an Ext-transfer result from rational G-modules to rational H-modules. In case G = GLn, these results can be illustrated in terms of classical Schur algebras. In that case, we establish another category equivalence, this time between the module category for a Schur algebra and the module category for a union of blocks for a natural quotient of a larger Schur algebra. This category equivalence provides a further Ext-transfer theorem from the original Schur algebra to the larger Schur algebra. This result, announced in [PS2, (6.4b)], extends to the category level the decomposition number method of Erdmann [E2]. Finally, we indicate (largely without proof) some natural variations to situations involving quantum groups and q-Schur algebras.