1. On the number of limit cycles which appear with the variations of the coefficients from an equilibrium point of focus or center type;Bautin;AMS Transl. Ser.,1962
2. Plane Algebraic Curves;Brieskorn,1986
3. Normal Forms and Bifurcation of Planar Vector Fields;Chow,1994
4. Methods of Bifurcation Theory;Chow,1982
5. Estimating limit cycles bifurcations from centers;Christopher,2006