1. D. R. Bates, Quantum Theory, vol. 3, p. 44, Academic Press, New York and London, 1962.
2. R. J. Glauber, Lectures in Theoretical Physics, vol. I. p. 315, Interscience Publishers Inc. New York, 1959; for reference see alsoB. J. Malenka, Phys. Rev.,95, 522, 1956. andH. S. Volk, Phys. Rev.,122, 931, 1961.
3. G. Parzen, Phys. Rev.,104, 835, 1956.
4. T. Tietz, Acta Phys. Hung.,11, 85, 1960.