Perturbations for massive gravity theories
  A theory of massive gravity depends on a non-dynamical 'reference metric'
f_{\mu\nu} which is often taken to be the flat Minkowski metric. In this paper
we examine the theory of perturbations on a background with metric g_{\mu\nu}
which does not coincide with the reference metric f_{\mu\nu}. We derive the
mass term for general perturbations on this background and show that it
generically is not of the form of the Fierz-Pauli mass term. We explicitly
compute it for some cosmological situations.
