Generalised Kostka-Foulkes polynomials and cohomology of line bundles on homogeneous vector bundles

Let G be a simple algebraic group and B a Borel subgroup. We consider generalisations of Lusztig's q-analogues of weight multiplicity, where the set of positive roots is replaced with the multiset of weights of a B-submodule N of an arbitrary finite-dimensional G-module V. The corresponding polynomials in q are called generalised Kostka-Foulkes polynomials (gKF). We prove vanishing theorems for the cohomology of line bundles on G x (B) N and derive from this a sufficient condition for the non-negativity of the coefficients of gKF. We also consider in detail the case in which V is the simple G-module whose highest weight is the short dominant root and N is the B-submodule whose weights are all short positive roots.