1. Kibble, T. W. B., J. Math. Phys. 2 (1961), 777.
2. Noether, E., Nachr. Ges. Wiss. Göttingen, Math.-Phys. K1. 1918, 235.
3. Yang, C. N. and R. L. Mills, Phys. Rev. 95 (1954), 191.
4. Landers, A. W., Jr., “Invariant Multiple Integrals in the Calculus of Variations,” in Contributions to th e Callculus of Variations, 1938–1941, 175 (Univ. Chicago Press, 1942).
5. Edelen, D. G. B., Arch. Rational Mech. Anal. 11 (1962), 117.