  Hamiltonian perturbation theory is used to analyse the stability of f(R)
models. The Hamiltonian equations for the metric and its momentum conjugate are
written for f(R) Lagrangian in the presence of perfect fluid matter. The
perturbations examined are perpendicular to R. As perturbations are added to
the metric and momentum conjugate to the induced metric instabilities are
found, depending on the form of f(R). Thus the examination of these
instabilities is a way to rule out certain f(R) models.
