Lykova О. B., and A. A. Boichuk. “Construction of Periodic Solutions of Non-Linear Systems in Critical Cases ”. Ukrains’kyi Matematychnyi Zhurnal 40, no. 1 (December 29, 1987): 62–69. Accessed February 17, 2026. https://umj.imath.kiev.ua/index.php/umj/article/view/9056.