1. Kadanoff, L. P. and Baym, G. (1962) Quantum Statistical Mechanics, Benjamin, New York.
2. Keldysh, L. V. (1965) Sov. Phys. JETP
20, 1018.
3. Haug, H. (ed.) (1988) Optical Nonlinearities and Instabilities in Semiconductors, Academic, San Diego, p. 53.
4. Schäfer, W. (1988) Optical Nonlinearities and Instabilities in Semiconductors (ed. H. Haug), Academic, San Diego, p. 133.
5. Schmitt-Rink, S. and Chemla, D. S. (1986) Phys. Rev. Lett., 57, 2752.