1. Theory of bifurcations of dynamic systems on the plane;Andronov,1973
2. On the stability of a single cycle;Cherkas;Diff. Uravnenija,1968
3. Methods of bifurcation theory;Chow,1982
4. Elementary graphics of cyclicity 1 and 2;Dumortier;Nonlinearity,1994
5. Nonlinear oscillation dynamical systems and bifurcation of vector fields;Guckenheimer,1983