SOKIL, B. I. On the construction of asymptotic approximations for a nonautonomous wave equation. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 12, p. 1714–1716, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5568. Acesso em: 5 dec. 2025.