1. E. Fermi, J. Orear, A.H. Rosenfeld and R.A. Schluter, Nuclear Physics, Revised Ed. (Univ. of Chicago 1950), Ch.III.
2. E. Merzbacher, Quantum Mechanics, Second Ed. (Wiley, New York 19 0) p. 65
3. M.M. Nieto, V.P. Gutschick, C. Bender, F. Cooper and D. Strottman, Phys. Lett. B (to be published).
4. P.B. Abraham and H.E. Moses, Phys. Rev. A22 (1980) 1333
5. M.M. Nieto, V.P. Gutschick, Phys. Rev. D23 (1981) 922; M.M. Nieto, Phys. Rev. D24 (1981) 1030