SUGIE, J.; YAMAOKA, N. Multilayer structures of second-order linear differential equations of Euler type and their application to nonlinear oscillations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 58, n. 12, p. 1704–1714, 2006. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3566. Acesso em: 27 apr. 2025.