BIGUN, Ya. I. Existence of a solution and averaging of nonlinear multifrequency problems with delay. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 59, n. 4, p. 435–446, 2007. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3318. Acesso em: 5 dec. 2025.