VALEEV, K. G.; VAZHGOVSKAYA, M. Ya. Investigation of stability in resonance cases . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 23, n. 1, p. 63–70, 1970. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8481. Acesso em: 6 dec. 2025.