1. C. Romero, J.B. Fonseca-Neto, M.L. Pucheu, Found. Phys. 42, 224 (2012) arXiv:1101.5333v1 [gr-qc]
2. S. Carroll, Spacetime and Geometry. An Introduction to General Relativity (Addison Wesley, San Francisco, 2004) p. 130
3. R. d’Inverno, Introducing Einstein’s Relativity (Oxford University Press, Oxford, 1992) p. 89
4. J.V. Narlikar, T. Padmanabhan, Phys. Rep. 100, 151 (1983)
5. L.S. Schulman, Techniques and Applications of Path Integration (J. Wiley, New York, 1981) pp. 88, 95, 96