1. Geometric Methods in Theory of Ordinary Differential Equations;Arnold,1983
2. Dynamical systems-I, ordinary differential equations;Arnold,1988
3. On the number of limit cycles which appear with the variation of the coefficients from an equillibrium position of focus or center type;Bautin;Math. USSR Sb.,1954
4. Local Theory of Nonlinear Analytic Ordinary Differential Equations;Bibikov,1979
5. Local orbital normal forms of vector fields in the plane;Bogdanov;Trudy Sem. Petrovsk.,1979