MITROPOLSKIY, Yu. A.; KHOMA, G. P. On the periodic solutions of the second-order wave equations. V. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 8, p. 1115–1121, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5908. Acesso em: 5 dec. 2025.