Phantom behaviour and growth index anomalous evolution in viable f(R)
  gravity models
  We present numerical calculation of the evolution of a background space-time
metric and sub-horizon matter density perturbations in viable $f(R)$ gravity
models of present dark energy and cosmic acceleration. We found that viable
models generically exhibit recent crossing of the phantom boundary $w_{DE}=-1$.
Moreover, 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.
