We give a necessary and sufficient condition for all solutions of a periodically forced Liénard system to be oscillatory. This paper also deals with the question when all trajectories keep on rotating around the origin. Several examples and global phase portraits are given to illustrate our results.
"Oscillatory solutions and rotatory solutions for a periodically forced Liénard system." Differential Integral Equations 10 (4) 717 - 738, 1997.