V. O. Er’omenko and A. M. Aliluiko, “Quasiperiodic solutions of degenerate linear systems of second-order ordinary differential equations”, Ukr. Mat. Zhurn., vol. 62, no. 6, pp. 773–783, Jun. 2010, Accessed: Apr. 09, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/2908