PETRYSHYN, R. I. Averaging method in some problems of the theory of nonlinear oscillations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 6, p. 801–810, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5474. Acesso em: 6 dec. 2025.