Static and spherically symmetric black holes in $f(R)$ theories
  We study some features of static and spherically symmetric solutions (SSS)
with a horizon in $f(R)$ theories of gravitation by means of a near-horizon
analysis. A necessary condition for an $f(R)$ theory to have this type of
solution is obtained. General features of the effective potential are deduced,
and it is shown that there exists a limit on the curvature at the horizon, in
both cases for any $f(R$). Finally, we calculate the expression for the energy
of the collision of two masive particles in the center of mass frame.
