Cosmological perturbations in the models of dark energy and modified
  gravity
  The quasi-static solutions of the matter density perturbation in various dark
energy models and modified gravity models have been investigated in numerous
papers. However, the oscillating solutions in those models have not been
investigated enough so far. In this paper, the oscillating solutions, which
have a possibility to unveil the difference between the models of the late-time
accelerated expansion of the Universe, are also mentioned by using appropriate
approximations.
