Linear growth in power law $f(T)$ gravity
  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.
