Dispersion in the growth of matter perturbations
  We consider the linear growth of matter perturbations on low redshifts in
modified gravity Dark Energy (DE) models where G_eff(z,k) is explicitly
scale-dependent. Dispersion in the growth today will only appear for scales of
the order the critical scale ~ \lambda_{c,0}, the range of the fifth-force
today. We generalize the constraint equation satisfied by the parameters
\gamma_0(k) and \gamma'_0(k) \equiv \frac{d\gamma(z,k)}{dz}(z=0) to models with
G_{eff,0}(k) \ne G. Measurement of \gamma_0(k) and \gamma'_0(k) on several
scales can provide information about \lambda_{c,0}. In the absence of
dispersion when \lambda_{c,0} is large compared to the probed scales,
measurement of \gamma_0 and \gamma'_0 provides a consistency check independent
of \lambda_{c,0}. This applies in particular to results obtained earlier for a
viable f(R) model.
