Asymptotic stability and periodic solutions of vector Li\'enard equations

We prove the asymptotic stability of the equilibrium solution of a class of vector Li\'enard equations by means of LaSalle invariance principle. The key hypothesis consists in assuming that the intersections of the manifolds in $\{\dot V = 0\}$ be isolated. We deduce an existence theorem for periodic solutions of periodically perturbed vector Li\'enard equations.