Kerf, E.A. De; Bäuerle, G.G.A.; Ten Kroode, A.P.E., eds. (1997). "Extensions of Lie algebras". Lie Algebras - Finite and Infinite Dimensional Lie Algebras and Applications in Physics. Studies in Mathematical Physics. Vol. 7. pp. 5–48. doi:10.1016/S0925-8582(97)80002-4. ISBN978-0-444-82836-1. The reference states the fact and that it is difficult to show. No further references are given. Expressions on a slightly different form can be found though in Tuynman & Wiegerinck (1987) and Bargmann (1954). Tuynman, G.M.; Wiegerinck, W.A.J.J. (1987). "Central extensions and physics". Journal of Geometry and Physics. 4 (2): 207–258. Bibcode:1987JGP.....4..207T. doi:10.1016/0393-0440(87)90027-1. Bargmann, V. (1954). "On unitary ray representations of continuous groups". Ann. of Math. 59 (1): 1–46. doi:10.2307/1969831. JSTOR1969831.
Kerf, E.A. De; Bäuerle, G.G.A.; Ten Kroode, A.P.E., eds. (1997). "Explicit construction of affine Kac-Moody algebras". Lie Algebras - Finite and Infinite Dimensional Lie Algebras and Applications in Physics. Studies in Mathematical Physics. Vol. 7. pp. 49–70. doi:10.1016/S0925-8582(97)80003-6. ISBN978-0-444-82836-1.
Kerf, E.A. De; Bäuerle, G.G.A.; Ten Kroode, A.P.E., eds. (1997). "Extensions of Lie algebras". Lie Algebras - Finite and Infinite Dimensional Lie Algebras and Applications in Physics. Studies in Mathematical Physics. Vol. 7. pp. 5–48. doi:10.1016/S0925-8582(97)80002-4. ISBN978-0-444-82836-1.
Kerf, E.A. De; Bäuerle, G.G.A.; Ten Kroode, A.P.E., eds. (1997). "Extensions of Lie algebras". Lie Algebras - Finite and Infinite Dimensional Lie Algebras and Applications in Physics. Studies in Mathematical Physics. Vol. 7. pp. 5–48. doi:10.1016/S0925-8582(97)80002-4. ISBN978-0-444-82836-1. The reference states the fact and that it is difficult to show. No further references are given. Expressions on a slightly different form can be found though in Tuynman & Wiegerinck (1987) and Bargmann (1954). Tuynman, G.M.; Wiegerinck, W.A.J.J. (1987). "Central extensions and physics". Journal of Geometry and Physics. 4 (2): 207–258. Bibcode:1987JGP.....4..207T. doi:10.1016/0393-0440(87)90027-1. Bargmann, V. (1954). "On unitary ray representations of continuous groups". Ann. of Math. 59 (1): 1–46. doi:10.2307/1969831. JSTOR1969831.
Kerf, E.A. De; Bäuerle, G.G.A.; Ten Kroode, A.P.E., eds. (1997). "Extensions of Lie algebras". Lie Algebras - Finite and Infinite Dimensional Lie Algebras and Applications in Physics. Studies in Mathematical Physics. Vol. 7. pp. 5–48. doi:10.1016/S0925-8582(97)80002-4. ISBN978-0-444-82836-1. The reference states the fact and that it is difficult to show. No further references are given. Expressions on a slightly different form can be found though in Tuynman & Wiegerinck (1987) and Bargmann (1954). Tuynman, G.M.; Wiegerinck, W.A.J.J. (1987). "Central extensions and physics". Journal of Geometry and Physics. 4 (2): 207–258. Bibcode:1987JGP.....4..207T. doi:10.1016/0393-0440(87)90027-1. Bargmann, V. (1954). "On unitary ray representations of continuous groups". Ann. of Math. 59 (1): 1–46. doi:10.2307/1969831. JSTOR1969831.