1. J.M. Jauch, Theory of the scattering operator, Helv. Phys. Acta. 31 (1958), 127–158.
2. T. Kato, Perturbation theory for linear operators, Springer, Berlin, (1966).
3. E. Prugovecki, Quantum mechanics in Hilbert space, Academic Press, New York, (1971).
4. K. Gustafson and K.Jörgens (unpublished); this observation is due principally to Jörgens.
5. K.Gustafson and G.Johnson, On the absolutely continuous subspace of a self-adjoint operator, Helv. Phys. Acta. (to appear).