Khoma, N. H. “Linear Periodic Boundary-Value Problem for a Second-Order Hyperbolic Equation. II. Quasilinear Problem”.
Ukrains’kyi Matematychnyi Zhurnal 50, no. 12 (December 25, 1998): 1680–1685. Accessed April 14, 2025.
https://umj.imath.kiev.ua/index.php/umj/article/view/4787.