1. Henneaux, M. (1985).Phys. Rep.,126, 1, and references therein.
2. Weitzenböck, R. (1923).Invarianten Theorie (Nordhoff, Gronigen).
3. Davies, E. T., and Yano, K. (1975). InConvegno Internazionale Celebrativo della Nascita di Tulli Levi-Civita (Atti dei Convegni Lincei, Accademia Nazionale di Lincei, Rome).
4. Kobayashi, S., and Nomizu, K. (1963).Foundations of Differential Geometry (Interscience, London/New York), vol. 1.
5. Bott, R., and Milnor, J. (1958).Bull. Am. Math. Soc.,64, 87. Kervair, M. (1958).Proc. Nat. Acad. Sci.,44, 286.