1. A. Einstein, The Meaning of Relativity (Princeton University Press, Princeton, 2004)
2. L.D. Landau, E.M. Lifschitz, Field Theory, Theoretical Physics, vol. 2 (Addison-Wesley, New York, 1957)
3. C.W. Misner, K.S. Thorne, J.A. Wheeler, Gravitation, 1st edn (W.H. Freeman, 1973)
4. R.M. Wald, General Relativity, 1st edn (University of Chicago Press, 1984)
5. C. Cremaschini, M. Tessarotto, Synchronous Lagrangian variational principles in general relativity. Eur. Phys. J. Plus 130, 123 (2015)