Khoma, N. H. “Linear Periodic Boundary-Value Problem for a Second-Order Hyperbolic Equation. I”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 11 (November 25, 1998): 1537–1544. Accessed April 11, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4806.