1. De Witt, B. S. (1964).Relativity, Groups, and Topology. Gordon and Breach, New York.
2. Penrose, R. (1970). InBeyond Quantum Theory, E. Bastin, ed. Cambridge University Press, New York.
3. Penrose, R., and Rindler, W. (1984).Spinors and Time-Space. Cambridge University Press, New York.
4. v. Weizsäcker, C. F. (1955).Zeitschrift für Naturforschung 13a, 245.