L. V. Ovsyannikov, Group Analysis of Differential Equations [in Russian], Nauka, Moscow (1978).
V. N. Shapovalov and G. G. Ékle, Algebraic Properties of the Dirac Equation [in Russian], Kalm. Univ., Élista (1972).
W. Miller, Symmetry and Separation of Variables [Russian translation], Mir, Moscow (1981).
E. L. Kalnins, W. Miller, Jr., and G. C. Williams, “Matrix operator symmetries of the Dirac equation and separation of variables,” J. Math. Phys.,27, No. 7, 1893–1900 (1986).
V. I. Fushchich and A. G. Nikitin, Symmetry of the Equations of Quantum Mechanics [in Russian], Nauka, Moscow (1990).
W. Killing, “Über die grundlagen der geometrie,” J. Reine Angew. Math.,109, 121–186 (1892).
N. Kh. Ibragimov, Transformation Groups in Mathematical Physics [in Russian], Nauka, Moscow (1983).
M. Walker and R. Penrose, “On quadratic first integrals of the geodesic equation for type 22 spacetimes,” Commun. Math. Phys.,18, No. 4, 265–274 (1970).
A. G. Nikitin and A. I. Prilipko, “Generalized Killing tensors and the symmetry of the Klein-Gordon-Fochs equation,” Preprint, Akad. Nauk UkrSSR, Inst. Matem., 90.26, 2–60 Kiev (1990).
G. H. Katrin and T. Levin, “Quadratic first integrals of the geodesies in spaces of constant curvature,” Tensor,16, No. 1, 97–103 (1965).
V. G. Bagrov, B. F. Samsonov, A. V. Shapovalov, and I. V. Shirokov, “Commutative subalgebras of symmetry operators of the wave equation containing second-order operator, and separation of variables,” Preprint, Sib. Br., Akad. Nauk SSSR, Tomsk. Nauch. Tsentr., 90.27, 3–60, Tomsk (1990).