1. Schneider, P., Ehlers, J., and Falco, E. E. (1992). Gravitational lenses, Springer-Verlag, Berlin.
2. Will, C. M. (1993). Theory and Experiments in Gravitational Physics, Cambridge University Press, Cambridge.
3. Viergutz, S. U. (1993). Aston. and Astrophys. 272, 355.
4. Bardeen, J. M. (1973). In: Black Holes, C. de Witt and B. S. de Witt, ed. NY, Gordon & Breach, 215.
5. Falcke, H., Melia F., and Agol, E. (1999). ApJ Letters 528, L13.