Instability of solutions for certain nonlinear vector differential equations of fourth order. Neliniini Kolyvannya, [S. l.], v. 12, n. 1, p. 120–129, 2009. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/513. Acesso em: 21 jan. 2026.