[1] J. R. Graef. On the Generalized Lienard equation with negative damping. J. Diff. Eqs12 (1972), 34~62.
[2] .G. Villari, On the qualitative behaviour of solutions of Lienard equation. J. Diff Eqs.,67 (1987), 269-27.
[3] Han Maoan, Periodic solution. unbounded solution and oscillation solution aboutequation Nalijillg University Maihenlaiical Biquarical Biquarterly 1(1984). 89-101. (in Chinese)
[4] Sun Jitao. On sufficient and necessary condition of solution to oscillate abount system kNanjing University Maithemtical Biquarteyly, 9 (1992). 113 -119. (in Chinese)
[5] Lin Jibin. A class of limit cycle distribution of plane cubic system, Scientia Sinica (SeriesA), 7 (1984). 586-596. (in Chinese)
[6] A. Sandquist and K. M. Andersen, A necessary and sufficient condition for the existenceof unique nontrivial periodic solution to a class of equations of Lienard type, J. Diff.Eqs. 46 (1982). 356-378.
[7] Wang Xian, Some remarks on the limit cycle for equation of Lienard type. Chin. Quar. J.Math.. 5 (1990). 122-130. (in Chinese)
[8] Sun Jiato. On the boundedness of solutions and existence of limit cycle of Lienard typeequation. Appl. Math. J. Chinese Unly. Ser. A, 7 (1992), 184-191. (in Chinese)
[9] Han Maoan. Global property of diffrential equation with multiple singular points. ActaMathematica Scientia. 33 (1990). 684-693. (in Chinese)
[10] Sun Jitao On the qualitative behaviour of solution of generalized Llienard equation. ActaMathematica Sclenlia 14 (1994). 90~95. (in Chinese)
[11] T Hara and T. Yoneyama, On the global center of generalized Lienard equation and itsapplication to stability problems, Funkical. Ekrac., 28 (1985), 171-192.
[12] G. Villari and F. Zanolin, On a dynamical system in the Lienard plane. necessary andsufficient conditions for the intersection with the vertical isocline and applications,FI,nkical. Ekvac., 33 (1990), 19-38.