1. P. Alsholm, Existence of limit cycles for generalized Liénard equations. J. Math. Anal. Appl., 1992, 171(1), 242-255.
2. L. A. Cherkas, Estimation of the number of limit cycles of autonomous systems. Differ. Uravn., 1977, 171, 779-802,translation in Differ. Equ., 1977, 13, 529-547.
3. M. P. Do Carmo, Differential geometry of curves and surfaces. Translated from the Portuguese. Prentice-Hall, Inc., Englewood Cliffs, N. J., 1976.
4. G. F. D. Duff, Limit-cycles and rotated vector fields. Ann. of Math. 1953, 57, 15-31.
5. F. Dumortier, J. Llibre and J. C. Artés, Qualitative theory of planar differential systems. Universitext. Springer Verlag, Berlin, 2006.