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.

中文翻译：

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希尔伯特空间中二阶微分方程的周期解

我们用下/上解法证明了一些无限维系统周期解的存在。良序和非良序情况都被处理，从而将标量方程的一些完善的结果推广到系统。