Lipchan, V. V. “On the Existence of an Invariant Manifold for Weakly Nonlinear Matrix Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, vol. 48, no. 3, Mar. 1996, pp. 422-4, https://umj.imath.kiev.ua/index.php/umj/article/view/5340.