1. After this was written, we learned thatJ. M. Cook had already treated the caseN=2 in the same spirit (see 1965 Cargèse Lectures in Theoretical Physics, edited byF. Lurcat. New York: Gordon & Breach 1967).
2. Kato, T.: Trans. Am. Math. Soc.70, 195 (1951).
3. Nelson, E.: Operator differential equations, Lemma 12.1, mimeographed lecture notes. Princeton University 1964.
4. Ruelle, D., unpublished.
5. Essentially we followJauch, J. M., Helv. Physica Acta31, 661 (1958), but we prefer a different definition of theS-operator, due toBerezin, F. A., L. D. Faddeev, andR. A. Minlos, Proceedings of the Fourth All-Union Mathematical Conference, held in Leningrad 1961.