Spotting deviations from R^2 inflation
  We discuss the soundness of inflationary scenarios in theories beyond the Starobinsky model, namely a class of theories described by arbitrary functions of the Ricci scalar and the K-essence field. We discuss the pathologies associated with higher-order equations of motion which will be shown to constrain the stability of this class of theories. We provide a general framework to calculate the slow-roll parameters and the corresponding mappings to the theory parameters. For paradigmatic gravitational models within the class of theories under consideration we illustrate the power of the Planck/Bicep2 latest results to constrain such gravitational Lagrangians. Finally, bounds for potential deviations from Starobinsky-like inflation are derived. 
Alvaro de la Cruz-Dombriz (ACGC, U. of Cape Town), Emilio Elizalde   (ICE-CSIC/IEEC), Sergei D. Odintsov (ICE-CSIC/IEEC and ...
gr-qc hep-th