Résumé
We study the growth of matter density perturbations delta_m for a number ofviable f(R) gravity models that satisfy both cosmological and local gravityconstraints, where the Lagrangian density f is a function of the Ricci scalarR. If the parameter m=Rf_{,RR}/f_{,R} today is larger than the order of10^{-6}, linear perturbations relevant to the matter power spectrum evolve witha growth rate s=d (ln delta_m)/d (ln a) (a is the scale factor) that is largerthan in the LCDM model. We find the window in the free parameter space of ourmodels for which spatial dispersion of the growth index gamma_0= gamma(z=0) (zis the redshift) appears in the range of values 0.40< gamma_0<0.55, as well asthe region in parameter space for which there is essentially no dispersion andgamma_0 converges to values around 0.40