1. T. Morita, Progr. Theor. Phys.,23, 829 (1960); T. Morita and K. Hiroike, Progr. Theor. Phys.,25, 537 (1961).
2. N. N. Bogolyubov, Dynamic-Theory Problems in Statistical Physics [in Russian], GITTI, Moscow (1946).
3. É. A. Arinshtein and B. G. Abrosimov, Zh. Strukt. Khim.,9, 1064 (1968).
4. B. G. Abrosimov and É. A. Arinshtein, Zh. Fiz. Khim.,10, 320 (1969).
5. C. J. Pings, Molec. Phys.,12, 501 (1967).