1. C. Eckart, Phys. Rev.,35, 1303 (1930).
2. E. T. Whittaker and G. N. Watson, A Course of Modern Analysis, Cambridge (1965).
3. R. G. Newton, Scattering Theory of Waves and Particles, New York (1966).
4. G. Bencze, Commentat. Phys. Math.,31, 1 (1966).