1. T. Matsubara, Prog. Theor. Phys. 14, 35 (1955).
2. P. C. Martin and J. Schwinger, Phys. Rev. 115, 1342 (1959).
3. A. A. Abrikosov, L. P. Gorkov, and I. E. Dzyaloshinskii, Methods of Quantum Field Theory in Statistical Physics, (Dover, New York, 1963).
4. L. P. Kadanoff and G. Baym, Quantum Statistical Mechanics, (Benjamin, New York, 1962).
5. P. Nozières, Theory of Interacting Fermi Systems, (Benjamin, New York, 1964).