Stability investigations of de Sitter inflationary solutions in power-law extensions of the Starobinsky model

In this paper, we would like to examine whether stable de Sitter inflationary solutions appear within power-law extensions of the Starobinsky model. In particular, we will address general constraints for the existence along with the stability of de Sitter inflationary solutions in a general case involving not only the Starobinsky R² term but also an additional power-law Rⁿ one. According to the obtained results, we will be able to identify which extension is more suitable for an early inflationary phase rather than a late-time cosmic acceleration phase. To be more specific, we will consider several values of n to see whether the corresponding de Sitter inflationary solutions are stable or not.