Evolution of Cosmological Perturbation in Reheating Phase of the   Universe
  The evolution of the cosmological perturbation during the oscillatory stage of the scalar field is investigated. For the power law potential of the inflaton field, the evolution equation of the Mukhanov's gauge invariant variable is reduced to the Mathieu equation and the density perturbation grows by the parametric resonance. 
Yasusada Nambu and Atsushi Taruya