We give the formulation of the gravitational lensing theory in the strong
field limit for a Schwarzschild black hole as a counterpart to the weak field
approach. It is possible to expand the full black hole lens equation to work a
simple analytical theory that describes at a high accuracy degree the physics
in the strong field limit. In this way, we derive compact and reliable
mathematical formulae for the position of additional critical curves,
relativistic images and their magnification, arising in this limit.Comment: 11 pages, 3 figure