1. J.A. de Azcarraga, J.M. Izquierdo, Lie Groups, Lie Algebras, Cohomology and Some Applications in Physics (Cambridge University Press, Cambridge, 1998). ISBN 0-521-46501. For the non-trivial factors in the composition laws see chapter 3.3, p. 163. For the role of the second cohomology group see p. 161. For the role of the third cohomology group and associativity see p. 186
2. J.A. de Azcarraga, J.M. Izquierdo, Lie Groups, Lie Algebras, Cohomology and Some Applications in Physics (Cambridge University Press, Cambridge, 1998). ISBN 0-521-46501. See p. 291 for the connection between the topological structure of the Galilei and Poincare groups and the existence of a simple covariant formulation
3. S. Hawking, Breakdown of predictability in gravitational collapse. Phys. Rev. D 10(14), 2460 (1976)
4. S. Hawking, Particle creation by black holes. Commun. Math. Phys. 43, 199 (1975)
5. N. Bogoliubov, On the theory of superfluidity. J. Phys. (USSR) 11, 23 (1947)