1. Hawking, S. W., and Ellis, G. F. R. (1973).The Large Scale Structure of Space-Time (Cambridge University Press, Massachusetts).
2. DeWitt, B. S. (1975).Phys. Rep.,C19 295.
3. Parker, L. (1979). InRecent Development in Gravitation (Plenum Press, New York).
4. Ford, L. H. (1975).Phys. Rev.,D11, 3370.
5. Schafer, G., and Dehnen, H. (1977).Astron. Astrophys.,54, 823; Schafer, G., and Dehnen, H. (1977).Astron. Astrophys.,61 671.