1. See for example R.G. Newton, “Scattering Theory of Waves and Particles”, Mc Graw-Hi1l,New York (1966).
2. E.E. Salpeter and H.A. Bethe, Phys. Rev. 84, 1232 (1951).
3. For a review and other references see N. Nakanishi, Prog. Theor. Phys., Suppl. 43, 1 (1969).
4. K. Kowalski, Phys. Rev. Lett. 15, 798 (1965).
5. H.P. Noyes, Phys. Rev. Lett. 15, 538 (1965).