Spherical Collapse in f(R) Gravity
  We use 1-dimensional numerical simulations to study spherical collapse in the
f(R) gravity models. We include the nonlinear coupling of the gravitational
potential to the scalar field in the theory and use a relaxation scheme to
follow the collapse. We find an unusual enhancement in density near the virial
radius which may provide observable tests of gravity. We also use the estimated
collapse time to calculate the critical overdensity $\delta_c$ used in
calculating the mass function and bias of halos. We find that analytical
approximations previously used in the literature do not capture the complexity
of nonlinear spherical collapse.
