Mitropolskiy, Yu. A. “On Construction of Asymptotic Solution of the Perturbed Klein-Gordon Equation”. Ukrains’kyi Matematychnyi Zhurnal 47, no. 9 (September 25, 1995): 1209–1216. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5520.