1. This problem has been debated, for instance, in the session on decaying systems at theSeventh International Colloquium on Group Theoretical Methods in Physics held in Austin, Texas, in 1978. See alsoG. Parravicini, V. Gorini andE. C. G. Sudarshan:Resonances, scattering theory and rigged Hilbert spaces, preprint University of Texas at Austin (1979) (submitted toJ. Math. Phys. (N. Y.)).
2. See,e.g.,M. Reed andB. Simon:Methods of Modern Mathematical Physics, Vol.4 (New York, N. Y., 1978), sect. XII.6, XIII.10, and references contained therein.
3. V. De Alfaro andT. Regge:Potential Scattering (Amsterdam, 1965).
4. K. O. Friedrichs:Commun. Pure Appl. Math.,1, 361 (1948);T. D. Lee:Phys. Rev.,95, 1329 (1954).
5. L. Fonda andG. C. Ghirardi:Nuovo Cimento A,67, 257 (1970).