Bonheure, Denis; De Coster, Colette Forced singular oscillators and the method of lower and upper solutions. (English) Zbl 1108.34033 Topol. Methods Nonlinear Anal. 22, No. 2, 297-317 (2003). Summary: We study the existence of positive periodic solutions of the second-order differential equation \[ u''+g(u)u'+f(t,u)=h(t) \] where \(f(t,\cdot)\) has a singularity of repulsive type at the origin. We use the method of lower and upper solutions. Cited in 38 Documents MSC: 34C25 Periodic solutions to ordinary differential equations 34B15 Nonlinear boundary value problems for ordinary differential equations PDFBibTeX XMLCite \textit{D. Bonheure} and \textit{C. De Coster}, Topol. Methods Nonlinear Anal. 22, No. 2, 297--317 (2003; Zbl 1108.34033) Full Text: DOI