1. T. W. Barrett:Nuovo Cimento,39, 116 (1977).
2. P. A. M. Dirac:Principles of Quantum Mechanics, 4th edition (Oxford, 1967), p. 87.
3. W. Heitler:The Quantum Theory of Radiation, 3rd edition (Oxford, 1954), p. 64.
4. L. I. Schiff:Quantum Mechanics, 3rd edition (New York, N. Y., 1968), p. 60.
5. W. H. Louisell:Quantum Statistical Properties of Radiation (New York, N. Y., 1973), p. 46.