@article {2021,
	title = {Periodic Solutions of Second-Order Differential Equations in Hilbert Spaces},
	volume = {18},
	year = {2021},
	month = {2021/09/07},
	pages = {223},
	abstract = {<p>We prove the existence of periodic solutions of some infinite-dimensional systems by the use of the lower/upper solutions method. Both the well-ordered and non-well-ordered cases are treated, thus generalizing to systems some well-established results for scalar equations.</p>},
	isbn = {1660-5454},
	url = {https://doi.org/10.1007/s00009-021-01857-8},
	author = {Alessandro Fonda and Giuliano Klun and Andrea Sfecci}
}
