Cosmological Perturbations in Extended Massive Gravity
  We study cosmological perturbations around self-accelerating solutions to two
extensions of nonlinear massive gravity: the quasi-dilaton theory and the
mass-varying theory. We examine stability of the cosmological solutions, and
the extent to which the vanishing of the kinetic terms for scalar and vector
perturbations of self-accelerating solutions in massive gravity is generic when
the theory is extended. We find that these kinetic terms are in general
non-vanishing in both extensions, though there are constraints on the
parameters and background evolution from demanding that they have the correct
sign. In particular, the self-accelerating solutions of the quasi-dilaton
theory are always unstable to scalar perturbations with wavelength shorter than
the Hubble length.
