1. T.D. Newton and E.P. Wigner, Rev. Mod. Phys. 21, 400 (1949).
2. J.M. Jauch and C. Piron, Helv. Phys. Acta 40 561 (1967).
3. W.O. Amrein, Helv. Phys. Acta 42 149 (1968).
4. See, for example, S. Schweber, “An Introduction to Relativistic Quantum Field Theory”, Harper & Row, New York, 1964.
5. M. Gell-Mann, Cal Tech Report CTSL-20 (1961), unpublished.