Periodic solutions of impulsive systems with a small delay in the critical case of second order. Neliniini Kolyvannya, [S. l.], v. 1, n. 1, p. 6–19, 1998. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/4. Acesso em: 22 jan. 2026.