BIGUN, Ya. I. Averaging method in multifrequency systems with delay. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 2, p. 299–303, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4963. Acesso em: 5 dec. 2025.