Relaxation of hierarchy in higher-dimensional Starobinsky model

Starobinsky model, which has a Ricci scalar squared term $R^{2}$ in its action, is one of the most promising inflation models from the viewpoint of Cosmic Microwave Background observations. However, it is well known that observational predictions of this model are quite sensitive to the existence of $R^{m}$ $(2<m)$ terms, whose absence is just assumed. In this paper, we clarify that the observational predictions of $D$-dimensional ($4<D$) extended Starobinsky model are less sensitive to such terms than those of the original 4-dimensional model.This result make it easier to construct Starobinsky-like models in higher dimensions.