1. DeWitt, B. (1967). Phys. Rev.
160, 1113.
2. Misner, C. W. (1972). In Magic without Magic: John Archibald Wheeler, J. R. Klauder, ed. (Freeman, San Francisco).
3. Wheeler, J. A. (1964). In Relativity, Groups and Topology, C. DeWitt and B. DeWitt, edd. (Gordon and Breach, New York).
4. Hawking, S. W. (1982). Pontif. Accad. Sci. Varia
48, 563. Hartle, J. B., and Hawking, S. W. (1983). Phys. Rev. D
28, 2690.
5. Macías, A., Obregón, O., and Ryan, M. P. (1987). Class. Quantum Grav.
4, 1477.