Gravitational Lensing by Spherical Lenses
  In this work we introduced a new proposal to study the gravitational lensing
theory by spherical lenses, starting from its surface mass density $\Sigma(x)$
written in terms of a decreasing function $f$ of a dimensionless coordinate $x$
on the lens plane. The main result is the use of the function $f(x)$ to find
directly the lens properties, at the same time that the lens problem is
described by a first order differential equation which encodes all information
about the lens. SIS and NIS profiles are used as examples to find their
functions $f(x)$. Using the Poisson equation we find that the deflection angle
is directly proportional to $f(x)$, and therefore the lens equation can be
written in terms of the function and the parameters of the lens. The critical
and caustic curves, as well as image formation and magnification generated by
the lens are analyzed. As an example of this method, the properties of a lens
modeled by a NFW profile are determined. Altough the puntual mass is
spherically symmetric, its mass density is not continuous so that its $f(x)$
function is discussed in the Appendix A.
