1. Qualitative Theory of Second-Order Dynamic Systems;Andronov,1973
2. On the behavior of integral curves of a differential equation;Berlinskii;Izv. Vyssh. Uchebn. Zaved. Mat.,1960
3. The number of limit cycles of certain polynomial differential equations;Blows,1984
4. Separatrix and limit cycles of quadratic systems and Dulac's theorem;Chicone;Trans. Amer. Math. Soc.,1983