1 Penrose R . Seminar on differential geometry. Princeton University Press , 1982
2 Duan Y S, Zhang J Y . Acta Physica Sinica, 1963; 19(11): 589
3 Duan Y S, Dong X G, Liu J C . Gen Rel Grav, 1988; 20(5): 485
4 Duan Y S, Wang Y T . Science in China, 1983; 4A: 343
5 Landau I D, Lifshitz E M . The classical theory of fields. Fourth revised English edition. Oxford : Pergamon Press, Ltd. 1987
6 Fock B A . Theory of spacetime and gravitation. Oxford : Pergamon Press , 1959
7 Komar A . Phys Rev, 1959; 113: 934
8 Ashtekar A, Winicour J . J Math Phys, 1982; 23(12): 2410
9 Duan Y S, Feng S S . Commu Theor Phys, 1996; 25: 99
10 Feng S S, Duan Y S . Gen Rel Grav, 1995; 27: 887
