1. L. V. Ovsyannikov,Group Analysis of Differential Equations [in Russian], Nauka, Moscow (1978).
2. N. Kh. Ibragimov,Transformation Groups in Methematical Physics [in Russian], Nauka, Moscow (1983).
3. P. J. Olver,Applications of Lie Groups to Differential Equations, Springer Verlag, New York (1986). [The pagination given in the text refers to the Russian translation published by Mir. Moscow (1989).]
4. V. N. Shapovalov,Izv. Vyssh. Uchebn. Zaved. Fiz., No. 6, 57 (1977).
5. N. Kh. Ibragimov and A. B. Shabat,Dokl. Akad. Nauk SSSR,244, 57 (1979).