1. Only a limited selection of references can be given here
2. Pauli, W.: In: Sommerfeld-Festschrift, Debye, P. (ed.), p. 30. Leipzig: Hirzel 1928
3. van Hove, L.: Physica21, 901 (1955);23, 441 (1957);25, 268 (1959)
4. Zwanzig, R.: In: Lectures in Theoretical Physics, Brittin, W., Dunham L. (eds.), Vol. 3, p. 135. New York: Wiley 1961; J. Chem. Phys.33, 1338 (1960); Physica30, 1109 (1964)
5. Ludwig, G.: Physica28, 841 (1962)