Lusztig’s q-analogue of weight multiplicity and one-dimensional sums for affine root systems

In this paper we complete the proof of the X = K conjecture, that for every family of nonexceptional affine algebras, the graded multiplicities of tensor products of “symmetric power” Kirillov-Reshetikhin modules known as one-dimensional sums, have a large rank stable limit X that has a simple expression (called the K-polynomial) as nonnegative integer combination of Kostka-Foulkes polynomials. We consider a subfamily of Lusztig’s q-analogues of weight multiplicity which we call stable KL polynomials and denote by ∞KL. We give a type-independent proof that K = ∞KL. This proves that X = ∞KL: the family of stable one-dimensional sums coincides with family of stable KL polynomials. Our result generalizes the theorem of Nakayashiki and Yamada which establishes the above equality in the case of one-dimensional sums of affine type A and the Lusztig q-analogue of type A, where both are Kostka-Foulkes polynomials.