1. L.V. Ovsyannikov, Group Analysis of Differential Equations, Nauka, Moscow, 1978 (English trans., Academic Press, New York, 1982).
2. N.H. Ibragimov, Transformation Groups Applied to Mathematical Physics, Nauka, Moscow, 1983 (English trans., Boston, Reidel, 1985).
3. L.M. Berkovich, Method of factorization for differential operators, and its application to solutions ordinary linear differential equations with variable coefficients, autoreview of thesis, Ural State University, Sverdlovsk, 1967, p. 13.
4. L.M. Berkovich, Transformation of ordinary linear differential equations, Kujbyshev State University Publication, Kujbyshev, 1978 (in Russian).
5. L.M. Berkovich, Factorization and transformation of ordinary differential equations, Saratov State University Publication, Saratov, 1989 (in Russian).