1. F. Calogero andYu. A. Simonov:Phys. Rev. (in press).
2. L. Spruch:Minimum Principles in Scattering Theory, inLectures in Theoretical Physics, Boulder, Colo., 1961, edited byE. W. Brittin, W. B. Downs andJ. Downs (New York, 1962).
3. L. Spruch:Necessary conditions for the existence of bound states, inFew-Nucleon Problems, Proceedings of the Ninth Summer Meeting of Nuclear Physicists, Hercegnovi, 1964, edited byM. Cerineo, published by the Federal Nuclear Energy Commission of Yugoslavia, vol.1, p. 271. The possibility that this approach had been previously used by Svartholm in his thesis at Lund in 1945 is mentioned by Spruch (see p. 309).
4. R. Jost andA. Pais:Phys. Rev.,82, 840 (1951). See also:V. Bargmann:Proc. Acad. Sci. U.S.A.,38, 961 (1952).
5. J. Schwinger:Proc. Acad. Sci. U.S.A.,47, 122 (1961).