Lykova О. B. and A. A. . Boichuk, “Construction of periodic solutions of non-linear systems in critical cases ”, Ukr. Mat. Zhurn., vol. 40, no. 1, pp. 62–69, Dec. 1987, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/9056