1. B. S. DeWitt, “Quantum theory of gravity. I. The canonical theory,” Phys. Rev. 160, 1113 (1967).
2. P. W. Higgs, “Integration of secondary constraints in quantized general relativity,” Phys. Rev. Lett. 1, 373 (1958).
3. C. Kiefer, Quantum Gravity (Oxford University Press, Oxford, 2007).
4. J. B. Hartle, “Quantum cosmology: Problems for the 21st Century,” in Proceedings of the 11th Nishinomiya-Yukawa Symposium, Ed. K. Kikkawa, H. Kunitomo, and H. Ohtsubo (World Scientific, Singapore, 1998).
5. C. Kiefer, “Conceptual issues in quantum cosmology,” in Towards Quantum Gravity. Proceeding of the XXXV International Winter School on Theoretical Physics Held in Polanica, Poland, 2–11 February 1999, Ed. by J. Kowalski-Glikman (Lecture Notes in Physics, v. 541, Springer, Berlin, 2000).