1. W. Drechsler,Fortschr. Phys. 23, 607 (1975).
2. W. Drechsler, Wave equations for extended hadrons, inGroup Theoretical Methods in Physics (Fourth Intern. Colloquium, Nijmegen, 1975), A. Janner, T. Janssen, and M. Boon, eds.,Lecture Notes in Physics, Vol. 50 (Springer-Verlag, 1976), p. 37.
3. R. Utiyama,Phys. Rev. 101, 1597 (1956); T. W. B. Kibble,J. Math. Phys. 2, 212 (1961).
4. D. W. Sciama, inRecent Developments in General Relativity (Pergamon Press, London, 1962), p. 415.
5. C. N. Yang and R. L. Mills,Phys. Rev. 96, 191 (1954).