Linear growth in power law<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>f</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mi>T</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>gravity

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

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