1. M.D. Schwartz, Quantum field theory and the standard model (Cambridge University Press, NY, 2014)
2. R.M. Wald, General relativity (University of Chicago press, NY, 2010)
3. C. Rovelli, Quantum gravity (Cambridge University Press, NY, 2004)
4. N. E. Mavromatos, arXiv preprint arXiv:0708.2250 ( 2007)
5. M. Pospelov, Y. Shang, Phys. Rev. D 85, 105001 (2012)