Another proof of Joseph and Letzter's separation of variables theorem for quantum groups

Let g be a simple finite-dimensional complex Lie algebra and letG be the corresponding simply-connected algebraic group. A theorem of Kostant states that the universal enveloping algebra of g is a free module over its center. A theorem of Richardson states that the algebra of regular functions ofG is a free module over the subalgebra of regular class functions. Joseph and Letzter extended Kostant's theorem to the case of the quantized enveloping algebra of g. Using the theory of crystal bases as the main tool, we prove a quantum analogue of Richardson's theorem. From it, we recover Joseph and Letzter's result by a kind of “quantum duality principle”.