MitropolskiyYu. A., and KhomaG. P. “On the Periodic Solutions of the Second-Order Wave Equations. V”. Ukrains’kyi Matematychnyi Zhurnal 45, no. 8 (August 25, 1993): 1115–1121. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5908.