Practical solutions for perturbed f(R) gravity
  We examine the accuracy of the quasi-static approximation and a parametric
form commonly employed for evolving linear perturbations in f(R) gravity
theories and placing cosmological constraints. In particular, we analyze the
nature and the importance of the near horizon effects that are often neglected.
We find that for viable models, with a small present value of the scalaron
field, such corrections are entirely negligible with no impact on observables.
We also find that the one-parameter form, commonly used to represent the
modified equations for linear perturbations in f(R), leads to theoretical
systematic errors that are relatively small and, therefore, is adequate for
placing constraint on f(R) models.
