Periodic solutions of generalized Liénard equations (Q5899926)

First, the autor shows by means of a counterexample that the existence theorem for periodic solutions for the equation \(\ddot x+f(x,\dot x)\dot x+g(x)=0\) given by \textit{P. J. Ponzo} and \textit{N. Wax} [J. Math. Anal. Appl. 104, 117-127 (1984; Zbl 0618.34034)] is false. Next, he gives a correct version of such a theorem and an illuminating example.