1. Nonlinear superposition;Jones;J. Math. Anal. Appl.,1967
2. The center-focus problem and reversibility;Teixeira;J. Differential Equations,2001
3. Introduction to Nonlinear Differential and Integral Equations;Davis,1962
4. Vorlesungen über Continuierliche Gruppen mit geometrischen and Anderen Anwendungen;Lie,1893