Three Einstein rings: explicit solution and numerical simulation
  We investigated the effects of gravitational lensing for a system in which a
lens is a point mass and a homogeneous disc with a central hole. In such system
there is a variety of cases resulting in formation of one, two and three
Einstein rings. We found an explicit solution and considered conditions for
existence of the second Einstein ring arising on the disc. Numerical modelling
of the images was made for various ratios of the central mass to the disc one
and for various values of the disc surface density. We also analysed dependence
of the magnification factor on a source position for such system. The result of
our work can be used in search of astrophysical objects with a toroidal (ring)
structure.
