SokilB. I. “On the Construction of Asymptotic Approximations for a Nonautonomous Wave Equation”. Ukrains’kyi Matematychnyi Zhurnal 47, no. 12 (December 25, 1995): 1714–1716. Accessed September 1, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5568.