1. V.I. Arnold: ”Geometrical methods in the theory of ordinary differential equations”; Springer (Berlin) 1983
2. G.W. Bluman and J.D. Cole: ”Similarity methods for differential equations”; Springer (New York) 1974
3. G.W. Bluman and S. Kumei: ”Symmetries and differential equations”; Springer (New York) 1989
4. G.E. Bredon, ”Introduction to Compact Transformation Groups”, Academic Press, New York (1972)
5. A.D. Bruno: ”Normal forms”; Springer 1990