1. L. Leplae, F. Mancini, and H. Umezawa,Phys. Rep. 10C:151 (1974).
2. Y. Takahashi and H. Umezawa,Collect. Phenom. 2:55 (1975).
3. H. Umezawa, H. Matsumoto, and M. Tachiki,Thermo Field Dynamics and Condensed States (North-Holland, Amsterdam, New York, 1982).
4. M. Suzuki, inProgress in Quantum Field Theory, H. Ezawa and S. Kamefuchi, eds. (North-Holland, Amsterdam, 1986).
5. H. Matsumoto, in the same book as in Ref. 4.Progress in Quantum Field Theory, H. Ezawa and S. Kamefuchi, eds. (North Holland, Amsterdam, 1985).