1. B. S. DeWitt, Phys. Rev. 160, 1113 (1967); J. A. Wheeler, in Battelle Rencontres: 1967 Lectures in Mathematical Physics, ed. B. DeWitt and J. A. Wheeler (Benjamin, New York, 1968).
2. J. J. Halliwell, in Quantum Cosmology and Baby Universes, ed. S. Coleman, J.B. Hartle, T. Piran, and S. Weinberg (World Scientific, Singapore, 1991).
3. W. F. Blyth and C. J. Isham, Phys. Rev. 11, 768–778 (1975).
4. R. Laflamme and E. P. S Shellard, Phys.Rev. 35, 2315 (1987).
5. J. B. Hartle and S. W. Hawking, Phys. Rev. D 28, 2960 (1983).