1. Kaluza, T. (1921).Sitzungsber. Preuss. Akad. Wiss. Berlin, Phys. Math.,K1, 966; Klein, O. (1926).Z. Phys. 37, 895.
2. For a review of Kaluza-Klein theories see, for example T. Appelquist, A. Chodos, and P. G. O. Freund (1987).Modern Kaluza Klein Theories (Addison-Wesley, New York).
3. Green, M., Schwarz, J., and Witten, E. (1987).Superstring Theory (2 vols., Cambridge University Press, Cambridge).
4. Levi-Civita, T. (1917).Rend. Atti Acad. Naz. Lincei,26, 529.
5. Robinson, I. (1959).Bull. Akad. Pol.,7, 351.