On the periodic orbits of the third-order differential equation x‴−μx″+x′−μx=εF(x,x′,x″)x‴−μx″+x′−μx=εF(x,x′,x″)

In this paper we study the periodic orbits of the third-order differential equation x‴−μx″+x′−μx=εF(x,x′,x″)x‴−μx″+x′−μx=εF(x,x′,x″), where εε is a small parameter and the function FF is of class C2C2.