1. G.M. Bergman, The diamond lemma for ring theory, Adv. Math.
29 (1978), 178–218.
2. I.N. Bernstein, S.I. Gelfand, Tensor products of finite and infinite dimensional representations of semisimple Lie algebras, Compositio Math. 41 (1980), 245–285.
3. J. Bernstein, V. Lunts, A simple proof of Kostant’s theorem that U(g) is free over its center, Amer. J. Math.
118 (1996), 979–987.
4. W. Borho, J.-L. Brylinski, R. MacPherson, Nilpotent Orbits, Primitive Ideals and Characteristic Classes, Progr. Math. 78, Birkhäuser, Boston 1989.
5. W. Borho, J.-C. Jantzen, Über primitive Ideale in der Einhüllenden einer halbeinfachen Lie-Algebra, Invent. Math.
39 (1977), 1–53.