  We use the clustering properties of Luminous Red Galaxies (LRGs) and the
growth rate data provided by the various galaxy surveys in order to constrain
the growth index ($\gamma$) of the linear matter fluctuations. We perform a
standard $\chi^2$-minimization procedure between theoretical expectations and
data, followed by a joint likelihood analysis and we find a value of
$\gamma=0.56\pm 0.05$, perfectly consistent with the expectations of the
$\Lambda$CDM model, and $\Omega_{m0} =0.29\pm 0.01$, in very good agreement
with the latest Planck results. Our analysis provides significantly more
stringent growth index constraints with respect to previous studies, as
indicated by the fact that the corresponding uncertainty is only $\sim 0.09
\gamma$. Finally, allowing $\gamma$ to vary with redshift in two manners
(Taylor expansion around $z=0$, and Taylor expansion around the scale factor),
we find that the combined statistical analysis between our clustering and
literature growth data alleviates the degeneracy and obtain more stringent
constraints with respect to other recent studies.
