Cosmological perturbations in $F(R)$ gravity

The quasistatic solutions of the matter density perturbation in $F(R)$ gravity models have been investigated in numerous papers. However, the oscillating solutions in $F(R)$ gravity models have not been investigated enough so far. In this paper, the oscillating solutions are also examined by using appropriate approximations. And the behaviors of the matter density perturbation in $F(R)$ gravity models with singular evolutions of the physical parameters are shortly investigated as applications of the approximated calculations.