1. See, e.g, G.D. Mahan,Many-Particle Physics (Plenum, New York, 1981) Ch. 4.
2. R.P. Feynnian, Phys. Rev. 76 (1949)749; 767
3. A.L. Four and J.T. Walecka,Quantum Theory of Many-Particle Physics (McGraw-Hill, New York, 1971).
4. G.C. Wick, Phys. Rev. 80 (1950)268.
5. A.A. Abrikosov, L.P. Gorkov and I.E. Dzyalosbinski.Methods of Quantum Field Theory in Statistical Physics (Dover, New York, 1971).