Saddle point inflation from $f(R)$ theory

We analyse several saddle point inflationary scenarios based on power-law $f(R)$ models. We investigate inflation resulting from $f(R) = R + α_n M^{2(1-n)}R^n + α_{n+1}M^{-2n}R^{n+1}$ and $f(R) = \sum_n^l α_n M^{2(1-n)} R^n$ as well as $l\to\infty$ limit of the latter. In all cases we have found relation between $α_n$ coefficients and checked consistency with the PLANCK data as well as constraints coming from the stability of the models in question. Each of the models provides solutions which are both stable and consistent with PLANCK data, however only in parts of the parameter space where inflation starts on the plateau of the potential, some distance from the saddle. And thus all the correct solutions bear some resemblance to the Starobinsky model.

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