1. Theory of Bifurcations of Dynamical Systems on a Plane;Andronov,1973
2. Geometric Methods in the Theory of Ordinary Differential Equations;Arnold,1982
3. On the number of limit cycles which appear with the variation of coefficients from an equilibrium position of focus or center type;Bautin;Amer. Math. Soc. Transl.,1954
4. Bifurcation of critical periods for plane vector fields;Chicone;Trans. Amer. Math. Soc.,1989