1. E. Pessa:Lett Nuovo Cimento,15, 291 (1976).
2. Defined in I by formula (8) and similar for a; seeJ. D. Bjorken andS. D. Drell:Relativistic Quantum Fields (New York, N. Y., 1965), p. 27.
3. A similar situation arises in the Bose gas theory; see,e.g.,L. Landau andE. Lifshitz Physique statistique (Moscou 1967), p. 286;A. A. Abrikosov, L. P. Gor’kov andI. Ye. Dzyaloshinskii:Quantum Field Theoretical Methods in Statistical Physics Chap. I (Oxford, 1965).
4. N. Bogoliubov:Journ. Phys. (U.S.S.R.),9, 23 (1947);Sov. Phys., JETP,7, 51 (1958);Nuovo Cimento,7, 794 (1958); the use of Bogoliubov transformations in our problem has been introduced byL. Parker:Phys. Rev.,183, 1057 (1969); later calculations making use of the Bogoliubov transformations are those ofYa. B. Zel’dovich andA. A. Starobinskij;Scv. Phys. JETP,34, 1159 (1972);B. L. Hu, S. A. Fulling andL. Parker:Phys. Rev. D,8, 2377 (1973);M. Castagnino, A Verbeure andR. A. Weder:Nuovo Cimento,26 B, 396 (1975); see also for a mathematical characterizationF. A. Berezin:The Method of Second Quantization (New York, N. Y., 1966);A. Van Daele andA. Verbeure Comm. Math. Phys.,20, 268 (1971).
5. See,e.g.,S. Gasiorowicz:Elementary Particle Physics (New York, N. Y., 1966), p. 15.