B. I. Sokil, “Construction of one-frequency solutions of boundary-value problems for a nonautonomous wave equation”, Ukr. Mat. Zhurn., vol. 46, no. 9, pp. 1275–1279, Sep. 1994, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5646