1. See e.g., reviews:Fulton F., Rohrlich R., Witten L., Rev. Mod. Phys.34 (1962), 442;
2. Kastrup H. A., in Particles, Currents, and Symmetries. (Ed. P. Urban) Springer Verlag, Wien 1968.
3. Carruthers P., Phys. Reports1C (1971), 1;
4. Niederle J., in Proceedings of the VIIIth Winter School of Theoretical Physics in Karpacz, Vol. II., p. 44, Acta Universitatis Wratislaviensis, No. 164, Wrocław, 1972;
5. Barut A. O., in Proceedings of the Symposium on De Sitter and Conformal Groups, p. 3, University of Colorado, Summer 1970.