1. Martin P.C., Schwinger, J.: Phys. Rev.115, 1342 (1959)
2. Kadanov, L.P., Martin, P.C.: Phys. Rev.124, 670 (1961)
3. Kadanov, L.P., Baym, G.: Quantum statistical mechanics. New York: Benjamin 1962
4. Keldysh, L.V.: JETP Lett.20, 1018 (1965)
5. Abrikosov, A.A., Gorkov, L.P., Dzyaloshinski, I.E.: Quantum field the-oretical methods in statistical physics. Oxford: Pergamon 1965