1. J. Dixmier, Enveloping Algebras, North Holland, New York, 1977.
2. I. N. Bernstein, I. M. Gel’fand and S. I. Gel’fand, Funkts. Anal. Prilozh. 5 (1) (1971), 1; English translation: Funct. Anal. Appl. 5 (1) (1971), 1.
3. M. Bauer, P. Di Francesco, C. Itzykson and J. B. Zuber, Phys. Lett. 260B (1991), 323; Nucl. Phys. B362 (1991), 515.
4. P. Di Francesco and P. Mathieu, Phys. Lett. 278B (1992), 79.
5. A. Kent, Phys. Lett. 273B (1991), 56; Phys. Lett. 278B (1992), 443.