1. J. A. de Wet,Proc. Camb. Phil. Soc. 70, 485 (1971).
2. J. A. de Wet,Proc. Camb. Phil. Soc. 74, 149 (1973).
3. T. Kahan,Theory of Groups in Classical and Quantum Physics (Oliver and Boyd, Edinburgh, 1965), Part I, Ch. III.
4. P. Roman,Theory of Elementary Particles (North-Holland, Amsterdam, 1964), p. 88.
5. E. Wigner,Phys. Rev. 51, 106 (1937).