Additional invariance of the Kemmer–Duffin and Rarita–Schwinger equations

Additional (implicit) symmetry of the Kemmer–Duffin, Rarita–Schwinger, and Dirac equations is established. It is shown that the invariance algebra of the Kemmer–Duffin equation is a 34-dimensional Lie algebra containing the algebra of $SU(3)$ as a subalgebra, and that the Rarita–Schwinger equation is invariant under a 64-dimensional Lie algebra including the subalgebra $O(2,4)$. The explicit form of the operator that reduces the Rarita–Schwinger equation to diagonal form is found and also that of the operator that transforms the Kemmer–Duffin equation into the Tamm–Sakata–Taketani equation. The algebra of the additional invariance of the Dirac and Tamm–Sakata–Taketani equations in the class of differential operators is found.