1. L. D. Faddeev, in: Proc. International Conference on Mathematical Problems in Quantum Field Theory and Quantum Statistics [in Russian], Nauka (1975), p. 218.
2. N. Kh. Ibragimov, Lie Groups in Some Problems of Mathematical Physics [in Russian], NGU, Novosibirsk (1972).
3. L. V. Ovsyannikov, Group Analysis of Differential Equations [in Russian], Nauka (1978).
4. S. A. Vladimirov, Symmetry Groups of Differential Equations and Relativistic Fields [in Russian], Atomizdat (1979).