Subbоtіn V. F., “Theorems of existence of periodic solutions of differential equations with a delayed argument”, Ukr. Mat. Zhurn., vol. 18, no. 4, pp. 128–134, Jun. 1966, Accessed: Dec. 15, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/8324