1. M. Schőnberg, Nuovo Cimento 8, 817 (1951).
2. M. L. Goldberger and K. M. Watson, Phys. Rev. 136, B1472 (1964).
3. D. Pines, “The Many-Body Problem”, Benjamin, N.Y. (1962).
4. In general of course, any dissipation process presents this problem; see, e.g., M. L. Goldberger and K. M. Watson, “Collision Theory”, J. Wiley and Sons, Inc., N.Y. (1964).
5. C. A. Nicolaides, Phys. Rev. A6, 2078 (1972); Nucl. Inst. Methods 110, 231 (1973).