1. For reviews see e.g.: H. Umezawa, H. Matsumoto, M. Tachiki: Thermofield dynamics and condensed states, Amsterdam: North Holland 1982; E. Shuryak: Phys. Rep. 61 (1980) 72; N. Landsman, Ch. van Weert: Phys. Rep. 145 (1987) 142. This last article contains a very complete list of references to the original literature
2. For a recent review, see e. g.: J. Cleymans, R. Gavai, E. Suhonen: Phys. Rep. 130, (1986) 218
3. A clear derivation of these rules can be found in N. Landsman, Ch. van Weert, Thermofield dynamics and condensed states. Amsterdam: North Holland 1982
4. R.L. Kobes, G.W. Semenoff: Nucl. Phys. B260 (1985) 714; B272 (1986) 329
5. T. Kinoshita: J. Math. Phys. 3 (1962) 650; T.D. Lee, M. Nauenberg: Phys. Rev. B133 (1964) 1549