V. V. Lipchan, “On the existence of an invariant manifold for weakly nonlinear matrix differential equations”, Ukr. Mat. Zhurn., vol. 48, no. 3, pp. 422–424, Mar. 1996, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5340