Friedmann equation and stability of inflationary higher derivative gravity

A stability analysis of the de Sitter universe in pure gravity theory is known to be useful in many aspects. We first show how to complete the proof of an earlier argument based on a redundant field equation. It is shown further that the stability condition applies to £0 Friedmann-Robertson-Walker spaces based on the nonredundant Friedmann equation derived from a simple effective Lagrangian. We show how to derive this expression for the Friedmann equation of pure gravity theory. This expression is also generalized to include scalar field interactions.