Analytic study of the effect of dark energy-dark matter interaction on
  the growth of structures
  Large-scale structure has been shown as a promising cosmic probe for
distinguishing and constraining dark energy models. Using the growth index
parametrization, we obtain an analytic formula for the growth rate of
structures in a coupled dark energy model in which the exchange of
energy-momentum is proportional to the dark energy density. We find that the
evolution of $f \sigma_8$ can be determined analytically once we know the
coupling, the dark energy equation of state, the present value of the dark
energy density parameter and the current mean amplitude of dark matter
fluctuations. After correcting the growth function for the correspondence with
the velocity field through the continuity equation in the interacting model, we
use our analytic result to compare the model's predictions with large-scale
structure observations.
