1. F. J. Belinfante and C. Möller:Mat. Fys. Medd.,28 (1954);
2. F. E. Low:Lecture Notes, Brandeis University, 1959;S. S. Schweber:An Introduction to Relativistic Quantum Field Theory, Chap. Xl (Evanston, Ill., 1961).
3. J. D. Dollard:Journ. Math. Phys.,7, 802 (1966).
4. K. O. Friedrichs:Perturbation of spectra in Hilbert space, Lect. in .1ppl. Math., vol. 3, chap. II (Providence, 1965).
5. L. Yan Hove:Physica,21, 901 (1955);22, 343 (1956).