Running Spectral Index from Inflation with Modulations
  We argue that a large negative running spectral index, if confirmed, might
suggest that there are abundant structures in the inflaton potential, which
result in a fairly large (both positive and negative) running of the spectral
index at all scales. It is shown that the center value of the running spectral
index suggested by the recent CMB data can be easily explained by an inflaton
potential with superimposed periodic oscillations. In contrast to cases with
constant running, the perturbation spectrum is enhanced at small scales, due to
the repeated modulations. We mention that such features at small scales may be
seen by 21 cm observations in the future.
