On existence of periodic solutions for a system of impulsive differential equations in the resonance case. Neliniini Kolyvannya, [S. l.], v. 2, n. 3, p. 306–313, 1999. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/65. Acesso em: 22 jan. 2026.