Eigenvalues of the Stewart-Lyth equation for inflation with a blue   spectrum
  By using the rather stringent nonlinear second order slow-roll approximation, we reconsider the nonlinear second order Abel equation of Stewart and Lyth. We determine a new blue eigenvalue spectrum. Some of the discrete values of the spectral index $n_s$ have consistent fits to the cumulative COBE data as well as to recent ground-base CMB experiments. 
Franz E. Schunck and Eckehard W. Mielke