1. Feng, B., Qian, M., Stability of homoclinic loops and bifurcation conditions of limit cycles,Acta Math. Sinica, 1985, 28(1): 53.
2. Han, M., Zhu, D.,Bifurcation Theory of Differential Equations (in Chinese), Beijing: Coal Industry Publishing House, 1994.
3. Han, M., On the number of limit cycles bifurcated from a homoclinic or heteroclinic loop,Science in China, Series A, 1993, 23(2): 113.
4. Rousarie, R., On the number of limit cycles which appear by perturbation of separatrix loop of planar vector fields,Bol. Soc.Brasil Mat.., 1986, 17: 67.
5. Guckenheimer, J., Holmes, P.,Nonlinear Oscillations, Dynamical Systems, and Bifurcations of Vector Fields, New York: Springer-Verlag, 1983.