Sokil, B.I. (1995) “On the construction of asymptotic approximations for a nonautonomous wave equation”, Ukrains’kyi Matematychnyi Zhurnal, 47(12), pp. 1714–1716. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5568 (Accessed: 10 April 2025).