1. L. Flamm, Phys. Z. 17, 448 (1916)
2. A. Einstein, N. Rosen, Phys. Rev. 48, 73 (1935)
3. M.S. Morris, K.S. Throne, AJP 6, 395 (1988)
4. M. Visser, Nucl. Phys. B 328, 203 (1989)
5. M. Visser, Lorentzian Wormholes: From Einstein to Hawking (American Inst. of Physics, 1995)