1. H. J. Lipkin:Lie Groups for Pedestrians (Amsterdam, 1965) andS. Kamefuchi andY. Takahashi:Nucl. Phys.,36, 177 (1962).
2. L. Van Hove:Mem. Acad. Roy. Soc. Belg. Cl. Sci.,6, 26 (1951).
3. G. Bisiacchi andP. Budini:Nuovo Cimento,44 A, 418 (1966).
4. See for exampleO. W. Greenberg andA. M. L. Messiah:Phys. Rev.,136 B, 248 (1964) and references quoted there.
5. H. S. Green:Phys. Rev.,90, 270 (1953).