Subbоtіn V. F. Theorems of existence of periodic solutions of differential equations with a delayed argument. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 18, n. 4, p. 128–134, 1966. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8324. Acesso em: 15 dec. 2025.