1. Brenig, W., Haag, R.: Fortschr. Phys.7, 183 (1959)
2. Buchholz, D.: Commun. math. Phys.42, 269–279 (1975)
3. Doplicher, S., Haag, R., Roberts, J. E.: Commun. math. Phys.13, 1 (1969)
4. Kato, T.: Perturbation Theory for Linear Operators, Chapter 10. Berlin-Heidelberg-New York: Springer 1966
5. Hepp, K.: Proc. Brandeis University Summer Institute in Theoretical Physics 1965 (eds. M. Chretién, S. Deses), Vol. 1, p. 137. New York: Gordon and Breach 1966