1. Formal scattering theory is explained in several well known text books. Original papers, where the single-channel case is extended to multichannel problems, are: H. Ekstein, Phys. Rev. 101, 880 (1956); Nuovo Cim. 4, 1017 (1956);
2. J. M. Jauch, Helv. Phys. Acta 31, 127 (1958); 31, 661 (1958).
3. Many general points are discussed in the article by W. Brenig and R. Haag, Fortschritte der Physik 7, 183 (1959). For further references see T. F. Jordan, J. Math. Phys. 3, 429 (1962) and the bibliography given in Ref. 18.
4. We assume 0a=fIpcPa(p)d3p, the momentum distribution 0a(p) being a tempered test function.
5. The reduced masses are u3=m1m2/(m1+m2) and M3=m3(m1+m2)/ /(m1+m2+m3).