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==极限环存在性理论==

==极限环存在性理论==

===庞加莱–本迪克松定理（Poincaré–Bendixson theorem===

===庞加莱–本迪克松定理（Poincaré–Bendixson theorem)===

===李纳定理（Liénard theorem）===

===李纳定理（Liénard theorem）===