1. G. Cosenza, A. Sciarrino andM. Toller: Internal Report No. 158, Istituto di Fisica «G. Marconi», Roma (1968).
2. G. Cosenza, A. Sciarrino andM. Toller: Trieste preprint IC/68/24. In this paper a complete list of references about the group-theoretical work can be found.
3. For a simple exposition of the relevant results of the group-theoretical approach to the scattering problem seeL. Bertocchi:Rapporteur's talk at the Heidelberg Conference on Elementary Particles (1967).
4. G. Cohen-Tannoudji, A. Morel andH. Navelet:Ann. of Phys.,46, 239 (1968).
5. D. Z. Freedman andJ. M. Wang:Phys. Rev.,153, 1596 (1967).