Departement de Mathematiques, Universite de Bordj Bou Arreridj, BordjBou Arreridj 34265, El anasser, Algeria; Departament de Matematiques, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Catalonia, Spain
Abstract:
We use three different results of the averaging theory of first order for studying the existence of new periodic solutions in the two Duffing differential equations $ddot y+ a sin y= b sin t$ and $ddot y+a y-c y^3=bsin t$, where $a$, $b$ and $c$ are real parameters.