Inflationary attractors in F(R) gravity

In this letter we shall demonstrate that the viable F(R) gravities can be classified mainly into two classes of inflationary attractors, either the R2 attractors or the α-attractors. To show this, we shall derive the most general relation between the tensor-to-scalar ratio r and the spectral index of primordial curvature perturbations ns, namely the r−ns relation, by assuming that the slow-roll condition constrains the values of the slow-roll...