1. Andronov, A.A., Leontovich, E.A., Gordon, I.I., Maier, A.G.: Qualitative Theory of Second-Order Dynamic Systems. Israel Program for Scientific Translations, Halsted press, New York (1973)
2. Andronov, A.A., Leontovich, E.A., Gordon, I.I., Maier, A.G.: Theory of Bifurcation of Dynamic Systems on a Plane. Israel Program for Scientific Translations, Halsted Press, New York (1973)
3. Artés, J.C., Kooij, R., Llibre, J.: Structurally stable quadratic vector fields. Mem. Am. Math. Soc. 134, 639 (1998)
4. Artés, J.C., Llibre, J.: Quadratic vector fields with a weak focus of third order. Publ. Mat. 41, 7–39 (1997)
5. Artés, J.C., Llibre, J., Rezende, A.C.: Structurally Unstable Quadratic Vector Fields of Codimension One. Springer, Berlin (2018)