The aim of this paper is to extend the theory of lower and upper solutions to the periodic problem associated with planar systems of differential equations. We generalize previously given definitions and we are able to treat both the well-ordered case and the non-well-ordered case. The proofs involve topological degree arguments, together with a detailed analysis of the solutions in the phase plane.

中文翻译：

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周期平面系统的有序和无序上下解

本文的目的是将上下限解的理论扩展到与微分方程的平面系统相关的周期问题。我们概括了先前给出的定义，并且能够处理井然有序的情况和井然有序的情况。证明涉及拓扑度论证，以及对相平面中解的详细分析。