An analytic calculation of the growth index for $f(R)$ dark energy model
  We derive the analytic formula of the growth index for $f(R)$ dark energy
model where the effect on the growth of matter density perturbation $\delta_m$
from modified gravity (MG) is encoded in the effective Newton coupling constant
$G_{\rm eff}$ in MG (or equivalently $g\equiv {G_{\rm eff}/ G}$). Based on the
analytic formula, we propose that the parameter $g$ can be directly figured out
by comparing the observed growth rate $f_g\equiv d\ln\delta_m/d\ln a$ to the
prediction of $f_g$ in general relativity.
