A new formula for weight multiplicities and characters

We describe a new formula for weight multiplicities and characters of semisimple Lie algebras. Our formula expresses these weight multiplicities as sums of positive rational numbers. In fact, the formula works more generally for the Jacobi polynomials of Heckman and Opdam, which are a one parameter deformation of these characters. As a consequence we prove that after suitable normalization, the coefficients of these polynomials are themselves polynomials in the parameter with positive integral coefficients. This generalizes our earlier joint work with F. Knop on Jack polynomials.