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