Y. A. Mitropolskiy, “On construction of asymptotic solution of the perturbed Klein-Gordon equation”, Ukr. Mat. Zhurn., vol. 47, no. 9, pp. 1209–1216, Sep. 1995, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5520