A new approach to a study of existence of periodic solutions and their construction for systems of differential equations. Neliniini Kolyvannya, [S. l.], v. 11, n. 1, p. 55–70, 2008. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/462. Acesso em: 21 jan. 2026.