be determined. It is easy to notice the similarity between tensor in dRGT and the tensor in Maheshwari work once is chosen. Also Ogievetsky–Polubarinov model mandates which means that in 4D, the variation is conformal. The dRGT massive fields split into two helicity-2 two helicity-1 and one helicity-0 degrees of freedom, just like those of Fierz-Pauli massive theory. However, the covariantization, together with the decoupling limit, guarantee that the symmetries of this massive theory are reduced to the symmetry of linearized general relativity plus that of massive theory, while the scalar decouples. If is chosen to be divergenceless, i.e. , the decoupling limit of dRGT gives the known linearized gravity. To see how that happens, expand the terms containing in the action in powers of , where is expressed in terms of fields like how is expressed in terms of The fields are replaced by: Then it follows that in the decoupling limit, i.e. when both the massive gravity Lagrangian is invariant under: as in Linearized general theory of relativity, as in Maxwell's electromagnetic theory, . In principle, the reference metric must be specified by hand, and therefore there is no single dRGT massive gravity theory, as the theory with a flat reference metric is different from one with a de Sitter reference metric, etc. Alternatively, one can think of as a constant of the theory, much like or . Instead of specifying a reference metric from the start, one can allow it to have its own dynamics. If the kinetic term for is also Einstein–Hilbert, then the theory remains ghost-free and we are left with a theory of massive bigravity, (or bimetric relativity, BR) propagating the two degrees of freedom of a massless graviton in addition to the five of a massive one. In practice it is unnecessary to compute the eigenvalues of (or ) in order to obtain the . They can be written directly in terms of as where brackets indicate a trace, It is the particular antisymmetric combination of terms in each of the which is responsible for rendering the Boulware–Deser ghost nondynamical. The choice to use or , with the identity matrix, is a convention, as in both cases the ghost-free mass term is a linear combination of the elementary symmetric polynomials of the chosen matrix. One can transform from one basis to the other, in which case the coefficients satisfy the relationship The coefficients