Growth factor in f(T) gravity
  We derive the evolution equation of growth factor for the matter over-dense
perturbation in $f(T)$ gravity. For instance, we investigate its behavior in
power law model at small redshift and compare it to the prediction of
$\Lambda$CDM and dark energy with the same equation of state in the framework
of Einstein general relativity. We find that the perturbation in $f(T)$ gravity
grows slower than that in Einstein general relativity if $\p f/\p T>0$ due to
the effectively weakened gravity.
