1. S.W. Hawking, G.F.R. Ellis, The Large Scale Structure of Space-Time (Cambridge University Press, 1973)
2. R.P. Geroch, G.T. Horowitz, Global structure of spacetimes, in General Relativity: An Einstein Centenary Survey (Cambridge University Press, 1979) pp. 212--293
3. M. Visser, Lorentzian Wormholes. From Einstein to Hawking (American Institute of Physics, 1996)
4. E.E. Flanagan, R.M. Wald, Phys. Rev. D 54, 6233 (1996)
5. C. Barcelo, M. Visser, Phys. Lett. B 466, 127 (1999)