1. C.J. Isham, Canonical quantum gravity and the problem of time, gr-qc/9210011 [ INSPIRE ].
2. B.S. DeWitt, The quantization of geometry, Gravitation: An Introduction to Current Research, in L. Witten ed., Wiley, New York U.S.A. (1962).
3. B.S. DeWitt, Quantum theory of gravity. I. The canonical theory, Phys. Rev. 160 (1967) 1113 [ INSPIRE ].
4. A.A. Grib, S.G. Mamayev and V.M. Mostepanenko, Vacuum Quantum Effects in Strong Fields, Friedmann Lab. Publ., St. Petersburg Russia (1994).
5. S.G. Mamaev, V.M. Mostepanenko and A.A. Starobinskii, Particle creation from the vacuum near a homogeneous isotropic singularity, Zh. Eksp. Teor. Fiz. 70 (1976) 1577 [Sov. Phys. JETP 43 (1976) 823].