1. H. Umezawa, S. Kamefuchi, Y. Tomozawa andM. Konuma:Nuovo Citnento,3, 772 (1956), which will be referred to as I.
2. G. KÄllén:Dan. Mat. Fys. Medd.,27, No. 12 (1953).
3. H. Lehmann:Nuovo Cimenta,11, 342 (1954).
4. S. Sakata, H. Umezawa andS. Kamefuchi:Phys. Rev.,84, 154 (1951);Prog. Theor. Phys.,7, 377 (1952).
5. L. D. Landau, A. A. Abrikosov andI. M. Halatnikov:Dohl. Akad. Nauk SSSR,95, 497, 733, 1177 (1954);96, 261 (1954); L. D. Landau and I. Pomerančuk: Dokl.Akad. Nauk SSSR,102, 489 (1955). In this connection see also R. P. Feynman:Proc. Fifth Rochester Conference,95 (1955).