  We consider predictions for structure formation from modifications to general
relativity in which the Einstein-Hilbert action is replaced by a general
function of the Ricci scalar. We work without fixing a gauge, as well as in
explicit popular coordinate choices, appropriate for the modification of
existing cosmological code. We present the framework in a comprehensive and
practical form that can be directly compared to standard perturbation analyses.
  By considering the full evolution equations, we resolve perceived
instabilities previously suggested, and instead find a suppression of
perturbations. This result presents significant challenges for agreement with
current cosmological structure formation observations.
  The findings apply to a broad range of forms of f(R) for which the
modification becomes important at low curvatures, disfavoring them in
comparison with the LCDM scenario. As such, these results provide a powerful
method to rule out a wide class of modified gravity models aimed at providing
an alternative explanation to the dark energy problem.
