Periodic solutions of Liénard equations with resonant isochronous potentials (Q1948761)

The authors prove different existence and multiplicity results for periodic solutions of a Liénard equation involving an isochronous potential. Thus, they generalize several results in the literature, starting with the pioneering papers by Lazer and Leach, Frederickson and Lazer, Fučik, Dancer, and others. The proofs are carried out by the use of topological degree methods. It is worth mentioning that, in some cases, the existence of infinitely many periodic solutions can be proved, as well.