1. See for instance Bjorken, J. D., Drell, S. D.: Relativistic Quantum Mechanics. New York: McGraw Hill Inc. 1964
2. Titchmarsh, E. C.: Proc. Roy. Soc.266 A, 33 (1962)
3. Veselić, K.: Commun. math. Phys.22, 27 (1971)
4. Hunziker, W.: unpublished report, available from the Institute of theoretical physics of the ETH
5. Prosser, R. T.: J. Math. Phys.4, 1048 (1964).