In this paper we study the following second-order periodic system: where has a singularity. Under some assumptions on the and by Ortega’ small twist theorem, we obtain the existence of quasi-periodic solutions and boundedness of all the solutions.
"Boundedness of Solutions for a Class of Second-Order Periodic Systems." Abstr. Appl. Anal. 2013 (SI28) 1 - 9, 2013. https://doi.org/10.1155/2013/267572