1. E. Inönü andE. P. Wigner:Proc. Nat. Acad. Sci.,39, 510 (1953).
2. W. T. Shaep:Racah algebra and the contraction of groups, AECL-1098 (Atomic Energy of Can. Ltd., Chalk River, Ont., 1960).
3. H. Bacry andJ. M. Lévy-Leblond:Journ. Math. Phys.,9, 1605 (1968).
4. V. Bargmann:Ann. Math.,59, 1 (1954).
5. See for instanceF. Gürsey:Introduction to the de Sitter group, inGroup Theoretical Concepts and Methods in Elementary Particle Physics, edited byF. Gürsey (New York, 1964).