In this paper, we study the case of single field inflation within the framework of modified gravity theory where the gravity part has an arbitrary form f(R). Via a conformal transformation, this case can be transformed into its Einstein frame where it looks like a two-field inflation model. However, due to the existence of the isocurvature modes in such a multi-degree-of-freedom (m.d.o.f.) system, the (curvature) perturbations are not equivalent in two frames, so despite of its convenience, it is illegal to treat the perturbations in its Einstein frame as the "real" ones as we always do for pure f(R) theory or single field with nonminimal coupling. Here by pulling the results of curvature perturbations back into its original Jordan frame, we show explicitly the power spectrum and spectral index of the perturbations in the Jordan frame, as well as how it differs from the Einstein frame. We also fit our results with the newest Planck data. Since there is large parameter space in these models, we show that it is easy to fit the data very well.
Qiu, T., & Xia, J. Q. (2015). Perturbations of single-field inflation in modified gravity theory. Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 744, 273–279. https://doi.org/10.1016/j.physletb.2015.03.061