1. 1. N. H. Ibragimov, "On the group classification of differential equations of second order," Dokl. AN SSSR, 183, No. 2, 274-277 (1968) (in Russian)
2. English translation: Sov. Math. Dokl., 9, No. 6, 1365-1369 (1968).
3. 2. V. I. Lahno, S. V. Spichak, V. I. Stohnii, Symmetry Analysis of Equations of Evolution Type [in Ukrainian], Mathematics and its Applications, Vol. 45, Institute of Mathematics of the Ukrainian National Academy of Sciences, Kyiv (2002).
4. 3. L. V. Ovsiannikov, Group Analysis of Differential Equations [in Russian], Nauka, Moscow (1978)
5. [in English] Acad. Press, New York (1982).