1. L. D. Landau, Zh. Eksp. Teor. Fiz.,11, 592 (1941); Collected Works [in Russian], Nauka (1969) (Vol. 1, p. 352 and Vol. 2 p. 32).
2. N. N. Bogolyubov, Izv. Akad. Nauk SSSR, Ser. Fiz.,11, 77 (1947); Selected Works, Vol. 2 [in Russian], Naukova Dumka (1969), p. 210.
3. S. T. Belyaev, Zh. Eksp. Teor. Fiz.,34, 433 (1958).
4. N. M. Hugenholtz and D. Pines, Phys. Rev.,116, 489 (1959).
5. R. P. Feynman, Phys. Rev.,94, 262 (1954).