Representation ring of Levi subgroups versus cohomology ring of flag varieties

Recall the classical result that the cup product structure constants for the singular cohomology with integral coefficients of the Grassmannian of r-planes coincide with the Littlewood-Richardson tensor product structure constants for GL(r). Specifically, the result asserts that there is an explicit ring homomorphism \phi: \Rep _{\poly}(GL(r)) to H^*(Gr(r, n)), where Gr(r, n) denotes the Grassmannian of r-planes in C^n and \Rep_{\poly} (GL(r)) denotes the polynomial representation ring of GL(r). This work seeks to achieve one possible generalization of this classical result for GL(r) and the Grassmannian Gr(r,n) to the Levi subgroups of any reductive group G and the corresponding flag varieties.