1. M. Gell-Mann, F. Low: Phys. Rev.95, 1300 (1954)
2. E.C.G. Stueckelberg, A. Petermann, Helv. Phys. Acta26, 499 (1953)
3. See, for instance F. Gürsey: In: Relativity, groups and topology. London: Gordon & Breach 1964; L. Pontryagin: Topologische Gruppen. Leipzig: Teubner 1957; B. Van der Waerden: Gruppen Theoret. Method in der Quantenmechanik. Berlin: Springer 1932
4. R.J. Crewther: In: Weak and electromagnetic interactions at high energies. eds. M. Lévy et al. Cargèse New York: Plenum (1975)
5. N. Bogoljubov, D. Shirkov: Dokl. Nauk. SSSR103, 203 (1955)