We provide for the first time the growth index of linear matter fluctuations of the power law $f(T) propto (-T)^{b}$ gravity model. We find that the asymptotic form of this particular $f(T)$ model is $gamma approx frac{6}{11-6b}$ which obviously extends that of the $Lambda$CDM model, $gamma_{Lambda}approx 6/11$. Finally, we generalize the growth index analysis of $f(T)$ gravity in the case where $gamma$ is allowed to vary with redshift.
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