1. L. V. Ovsyannikov, Group Analysis of Differential Equations [in Russian], Nauka, Moscow (1978); English transl., Acad. Press, New York (1982).
2. P. J. Olver, Applications of Lie Groups to Differential Equations (Grad. Texts Math., Vol. 107), Springer, New York (1986).
3. N. H. Ibragimov, Groups of Transformations in Mathematical Physics [in Russian], Nauka, Moscow (1983); English transl.: Transformation Groups Applied to Mathematical Physics, D. Reidel, Dordrecht (1985).
4. A. Tresse, Acta Math., 18, 1–88 (1894).
5. R. E. Popovich and V. N. Boiko, Vestn. Samar. Gos. Univ. Mat. Mekh. Fiz. Khim. Biol., No. 4, 49–56 (2000).