1. L. Fonda, G. C. Ghirardi, A. Rimini andT. Weber:Nuovo Cimento,15 A, 689 (1973).
2. See, for example,J. Lukierski: ICTP, Trieste preprint IC/72/128.
3. G. N. Fleming:Nuovo Cimento (in press).
4. R. G. Newton:Scattering Theory of Waves and Particles, Chap. 19 (New York, N. Y., 1965);R. G. Winter:Phys. Rev.,123, 1503 (1961).
5. L. Fonda andG. C. Ghirardi:Nuovo Cimento,6 A, 553 (1971).