1. Poincaré, H.: Sur les courbes définies par les équations différentielles (III). Journal de mathématiques pures et appliquées 4e série, tome 1, 167–244 (1885)
2. Andronov, A.A., Leontovich, E.A., Gordon, I.I., Maier, A.G.: Qualitative Theory of Second-Order Dynamic Systems. Nauka, Moscow (1966). Wiley, New York (1973)
3. Arnold, V.I., Ilyashenko, Y.S.: Ordinary differential equations. In: Advances in Science and Technology: Modern Problems in Mathematics: Fundamental Directions,vol. 1, pp. 7–149. VINITI, Moscow (1985). (in Russian)
4. Shilnikov, L.P., Shilnikov, A.L., Turaev, D.V., Chua, L.O.: Methods of Qualitative Theory in Nonlinear Dynamics, Part I, p. 419. Ijevsk, Moscow (2004). (in Russian)
5. Arnold, V.I.: Geometrical Methods in the Theory of Ordinary Differential Equations. Springer, Berlin (1988)