Periodic Solutions of a Class of Duffing DifferentialEquations

In this work we study the existence of new periodic solutions forthe well knwon class of Duffing differential equation of the form$x^{primeprime}+ c x^{prime}+ a(t) x +b(t) x^3 = h(t)$, where $c$is a real parameter, $a(t)$, $b(t)$ and $h(t)$ are continuous$T$--periodic functions. Our results are proved using threedifferent results on the averaging theory of first order.