Computation of the Power Spectrum in Chaotic $1/4 \lambda \phi^4$   Inflation
  The phase-integral approximation devised by Fr\"oman and Fr\"oman, is used for computing cosmological perturbations in the quartic chaotic inflationary model. The phase-integral formulas for the scalar power spectrum are explicitly obtained up to fifth order of the phase-integral approximation. As in previous reports [1-3], we point out that the accuracy of the phase-integral approximation compares favorably with the numerical results and those obtained using the slow-roll and uniform approximation methods. 
Clara Rojas and Victor M. Villalba
astro-ph.CO