MITROPOLSKIY, Yu. A. On construction of asymptotic solution of the perturbed Klein-Gordon equation. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 9, p. 1209–1216, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5520. Acesso em: 6 dec. 2025.