On harmonic elements for semi-simple Lie algebra

Let ${\goth g}$ be a semi-simple complex Lie algebra, ${\goth g}={\goth n^-}\oplus{\goth h}\oplus{\goth n}$ its triangular decomposition. Let $U({\goth g})$, resp. $U_q({\goth g})$, be its enveloping algebra, resp. its quantized enveloping algebra. This article gives a quantum approach to the combinatorics of (classical) harmonic elements and Kostant's generalized exponents for ${\goth \g}$. On the one hand, we give specialization results concerning harmonic elements, central elements of $U_q({\goth g})$, and the Joseph and Letzter's decomposition. For ${\goth g}={\goth sl}_{n+1}$, we describe the specialization of quantum harmonic space in the ${\math N}$-filtered algebra $U({\goth sl}_{n+1})$ as the materialization of a theorem of Lascoux-Leclerc-Thibon. This enables us to study a Joseph-Letzter decomposition in the algebra $U({\goth sl}_{n+1})$. On the other hand, we prove that highest weight harmonic elements can be calculated in terms of the dual of Lusztig's canonical base. In the simply laced case, we parametrize a base of $\n$-invariants of minimal primitive quotients by the set $\co$ of integral points of a convex cone.