Strong field limit of black hole gravitational lensing

We give the formulation of the gravitational lensing theory in the strong f ield limit for a Schwarzschild black hole as a counterpart to the weak fiel d approach. It is possible to expand the full black hole lens equation to w ork a simple analytical theory that describes the physics in the strong fie ld limit at a high accuracy degree. In this way, we derive compact and reli able mathematical formulae for the position of additional critical curves, relativistic images and their magnification, arising in this limit.