1. E.M. Lifshitz and L.P. Pitaevskii, Statistical Physics 3rd edition part 1, (Pergamon, Oxford, 1980) Chap. 75 and Chap. 77.
2. L. Pavesi et al., J. Phys. C Solid State Phys. 21, 1485 (1988).
3. H. Stolz and R. Zimmermann, phys. status solidi (b) 94, 135 (1979).
4. See e.g. C. Klingshirn and H. Haug, Phys. Reports 70, 315 (1981).
5. H. Haug and S. Schmitt-Rink, Prog. Quant. Electr. 9, 5 (1984).