Phantom boundary crossing and anomalous growth index of fluctuations in
  viable f(R) models of cosmic acceleration
  Evolution of a background space-time metric and sub-horizon matter density
perturbations in the Universe is numerically analyzed in viable $f(R)$ models
of present dark energy and cosmic acceleration. It is found that viable models
generically exhibit recent crossing of the phantom boundary $w_{\rm DE}=-1$.
Furthermore, it is shown that, as a consequence of the anomalous growth of
density perturbations during the end of the matter-dominated stage, their
growth index evolves non-monotonically with time and may even become negative
temporarily.
