Michele V. Bartuccelli, Jonathan H.B. Deane, Guido Gentile, Luke Marsh Invariant sets for the varactor equation (1537K, pdf) ABSTRACT. The differential equation $\ddot{x} + \gamma \dot{x} + x^\mu = f(t)$ with $f(t)$ positive, periodic and continuous is studied. After describing some physical applications of this equation, we construct a variety of invariant sets for it, thereby partitioning the phase plane into regions in which solutions grow without bound and also those in which bounded periodic solutions exist.