1. Hawking, S. W. & Penrose, R. Proc. R. Soc. A314, 529–548 (1969).
2. Hawking, S. W. & Ellis, G. F. R. The Large Scale Structure of Space-time (Cambridge University Press, 1973).
3. Misner, C. W. Phys. Rev. 186, 1328–1333 (1969).
4. Patton, C. M. & Wheeler, J. A. in Quantum Gravity: An Oxford Symposium (eds Isham, C. J., Penrose, R. & Sciama, D. W.) 538–605 (Clarendon, Oxford, 1975).
5. Parker, L. & Fulling, J. A. Phys. Rev. D 7, 2357–2374 (1973).