Perturbation theorems for a multifrequency system with pulses. Neliniini Kolyvannya, [S. l.], v. 18, n. 2, p. 280–289, 2015. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/786. Acesso em: 6 dec. 2025.