Self-accelerating massive gravity: Covariant perturbation theory

We undertake a complete and covariant treatment for the quadratic Lagrangian of all of the degrees of freedom of massive gravity with a fixed flat fiducial metric for arbitrary massive gravity parameters around any isotropic self-accelerating background solution. Generically, three out of four Stückelberg degrees of freedom propagate in addition to the usual two tensor degrees of freedom of general relativity. The complete kinetic structure typically is only revealed at an order in the graviton mass that is equivalent to retaining curvature terms in a locally flat expansion. These results resolve several apparent discrepancies in the literature where zero degrees of freedom propagate in either special cases or approximate treatments as well as decoupling limit analyses which attempt to count longitudinal degrees of freedom.