1. L. I. Schiff, Quantum Mechanics (McGraw Hill, New York, 1955)2nd Ed., p. 207.
2. E. Fermi, Notes on Quantum Mechanics ( University of Chicago Press, Chicago, 1961 ) p. 100.
3. H. A. Bethe, Ann. Physik, 5, 325 (1930).
4. H. A. Bethe and R. W. Jackiw, Intermediate Quantum Mechanics (Benjamin, New York, 1968), 2nd Ed., p. 305.
5. L. D. Landau and E. M. Lifshitz, Quantum Mechanics. Non-Relativistic Theory (Pergamon, London, 1965), 2nd Ed., p. 581