1. Qualitative Theory of Differential Equations;Zhifen,1985
2. Saddle quantities and applications;Joyal;J.D.E.,1989
3. An explicit expression of the first Liapunov and period constants with applications;Gasull;J.M.A.A.,1997
4. Liapunov approach to multiple Hopf bifurcation;Gobber;J.M.A.A.,1979
5. Theory of values of singular point in complex autonomous differential systems;Yirong;Science in China A,1989