Periodic solutions of forced Liénard-type equations

Using Mawhin's coincidence degree theory and Brosuk's theorem, some new sufficient conditions of the existence of periodic solutions for the well-known forced Liénard-type equations ẍ+f1(x)ẋ+f2(x)ẋ2+g(x)=e(t) are given. These new conditions are natural extension and generalization of the corresponding results already obtained results in the literature.