- 
								[1]
								R. Arnowitt, S. Deser and C. Misner, Phys. Rev., 116:1322 (1959) 
- 
								[2]
								R. A. d'Inverno and J. Smallwood, Phys. Rev. D, 22:1233 (1980) 
- 
								[3]
								J. Goldberg, Found. Phys., 14:1211 (1984) 
- 
								[4]
								H. Bondi, M. G. J. van der Burg, and A. W. K. Metzner, Proc. R. Soc. Lond. A, 269:21 (1962) 
- 
								[5]
								R. K. Sachs, Proc. R. Soc. Lond. A, 270:103 (1962) 
- 
								[6]
								A. Ashtakar, C. Beetle, and S. Fairhurst, Class. Quant. Grav., 16:L1 (1999) 
- 
								[7]
								A. Ashtekar and B. Krishnan, Living Rev. Relativ., 7:10 (2004) 
- 
								[8]
								B. Krishnan (2014) pp 527-555. In:Ashtekar A., Petkov V. (eds) Springer Handbook of Spacetime, (Springer, Berlin, Heidelberg) 
- 
								[9]
								A. Ashtekar, Phys. Rev. Lett., 57:2244 (1986) 
- 
								[10]
								R. d'Inverno and J. Vickers, Class. Quant. Grav., 12:753 (1995) 
- 
								[11]
								R. d'Inverno, P. Lambert, and J. Vickers, Class. Quant. Grav., 23:3747 (2006) 
- 
								[12]
								R. d'Inverno, P. Lambert, and J. Vickers, Class. Quant. Grav., 23:4511 (2006) 
- 
								[13]
								J. Goldberg, D. Robinson, and C. Soteriou, Class. Quant. Grav., 9:1309 (1992) 
- 
								[14]
								J. Goldberg and C. Soteriou, Class. Quant. Grav., 12:2779 (1995) 
- 
								[15]
								N. Bodendorfer, T. Thiemann, and A. Thurn, Class. Quant. Grav., 30:045001, 045002, 045003, 045004 (2013) 
- 
								[16]
								G. S. Hall, Symmetries and Curvature Structure in General Relativity, (World Scientific, Singapore, 2004) 
- 
								[17]
								J. Wang, Y. Ma, and X.-A. Zhao, Phys. Rev. D, 89:084065 (2014) 
- 
								[18]
								J. Wang and C.-G. Huang, Int. J. Mod. Phys. D, 25:1650100 (2016) 
- 
								[19]
								C.-G. Huang and J. Wang, Gen. Rel. Grav., 45:(2016) 
- 
								[20]
								J. Wang, C.-G. Huang, and L. Li, Chin. Phys. C, 40:(2016) 
- 
								[21]
								E. T. Newman and R. Penrose, J. Math. Phys., 3:566 (1962) 
- 
								[22]
								R. Basu, A. Chatterjee, and A. Ghosh, Class. Quant. Grav., 29:235010 (2012) 
- 
								[23]
								C. G. Huang and S. B. Kong, Commun. Theor. Phys., 68:227 (2017) 
- 
								[24]
								P. A. M. Dirac, Lectures on Quantum Mechanics, (Yeshiva University, New York, 1964)