1. Pinto-Neto N., Santini E.S.: Phys. Lett. A 315, 36–50 (2003)
2. Colistete R. Jr., Fabris J.C., Pinto-Neto N.: Phys. Rev. D 62, 083507–1 (2000)
3. Halliwell, J.J.: In: Coleman, S., Hartle, J.B., Piran, T., Weinberg, S. (eds.) Quantum Cosmology and Baby Universes. World Scientific, Singapore (1991)
4. Wiltshire, D.L.: In: Viswanathan, N., Woolcook, W.S. (eds.) Cosmology: the Physics of the Universe. World Scientific, Singapore (1996)
5. Kiefer C.: Quantum Gravity, 2nd edn. Oxford University Press, Oxford (2007)