GROMYAK, M. I. Construction of periodical solutions of the wave differential and integral-differential second order equations . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 40, n. 1, p. 48–53, 1987. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/9053. Acesso em: 5 dec. 2025.