1. Cartan, E., and Einstein, A. (1979).Letters of Absolution Parallelism (Princeton University Press, Princeton, NJ) and the references contained therein.
2. Einstein, A. (1956).The Meaning of Relativity (5th. ed., Princeton University Press Princeton, NJ).
3. Schrödinger, E. (1943).Proc. R. Irish Acad. A49 43, 135.
4. See for example, M. Tonnelat (1966).Einstein's Unified Field Theory (Gordon and Breech, NY).
5. Yang, C. N., and Mills, R. L. (1954).Phys. Rev. 96, 191.