Визначено умови для того, щоб початок був центром для класу кубічних диференційованих систем, що мають дві інваріантні прямі і одну інваріантну кубічну. Доведемо, що тонкий фокус $O (0,0)$ є центром тоді і тільки тоді, коли перші дві кількості Ляпунова зникають.

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