Degrees of Freedom in Massive Gravity
  We study in a systematic way a generic nonderivative (massive) deformation of general relativity using the Hamiltonian formalism. The number of propagating degrees of freedom is analyzed in a nonperturbative and background independent way. We show that the condition of having only five propagating degrees of freedom can be cast in a set of differential equations for the deforming potential. Though the conditions are rather restrictive, many solutions can be found. 
D. Comelli, M. Crisostomi, F. Nesti, L. Pilo
hep-th gr-qc hep-ph