1. See for example: W.H. Flygare: Molecular Structure and Dynamics (Prentice-Hall, Englewood Cliffs, N.J. 1978) p. 105
2. F. Urbach: Phys. Rev. 92,1324 (1953)
3. Y. Toyozawa: Prog. Theor. Phys. 20,53 (1958)
4. R. Kubo in: Fluctuation, Relaxation and Resonance in Magnetic Systems, ed. by D. ter Haar (Oliver and Boyd, Edinburgh 1965)
5. H. Sumi: J. Phys. Soc. Jpn. 32, 616 (1972)