1. General references on properties of thermodynamic Green’s functions are L. P. Kadanoff and G. Baym, Quantum Statistical Mechanics (W. A. Benjamin, New York, 1962) and A. Abrikosov, L. Gor’kov and I. Dzyaloshinski, The Methods of Quantum Field Theory in Statistical Physics (Prentice Hall, Englewood Cliffs, New Jersey, 1963).
2. An extensive discussion of the choice of ensemble is given by P. C. Hohenberg and P. C. Martin, Ann. Phys. (N.Y.) 34, 291 (1965). In this paper the reader will also find a derivation of the two-fluid model from the microscopic theory.
3. See for example, J. R. Schrieffer, Theory of Superconductivity (W. A. Benjamin, New York, 1964).
4. General review articles are J. Bardeen, in Quantum Theory of Atoms; Molecules and the Solid State, Editor P.-O. Löwdin (Academic Press, Inc., New York, 1966), p. 511; P. W. Anderson, Rev. Mod. Phys.
38, 298 (1966).
5. E.g. L. Landau and E. Lifshitz, Statistical Physics (Addison-Wesley, Reading, Mass., 1958).