MitropolskiyY. A. (1995) “On construction of asymptotic solution of the perturbed Klein-Gordon equation”, Ukrains’kyi Matematychnyi Zhurnal, 47(9), pp. 1209-1216. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5520 (Accessed: 27July2024).