The limit cycle problem of planar piecewise linear refracting systems has been solved except the focus–focus (FF) type. Here we concern this remaining FF type, and prove that such systems have at most one limit cycle and have 14 topologically different global phase portraits.

中文翻译：

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焦点-焦点类型的平面分段线性折射系统的极限环和全局动力学

平面分段线性折射系统的极限循环问题已得到解决，但焦点-焦点（FF）类型除外。在这里，我们关注剩余的FF类型，并证明此类系统最多具有一个极限环，并具有14个拓扑不同的全局相位肖像。