1. G. D. Mahan:Many Particle Physics (Plenum Press, New York, N.Y., 1981).
2. C. C. Lam, P. C. W. Fung andC. M. Cheng:Nuovo Cimento B,97, 75 (1987).
3. Seee.g. W. Louisell:Quantum Statistical Properties of Radiation (J. Wiley and Sons, New York, N.Y., 1973).
4. Lie Methods in Optics, edited byK. B. Wolf andN. Sanchez Mondragon (Springer Verlag, Wien, 1986);G. Dattoli, S. Solimeno andA. Torre:Phys. Rev. A,35, 1668 (1987).
5. B. L. Schumaker:Phys. Rep.,135, 317 (1986) and references therein.