Khoma, N. H. 1998. “Linear Periodic Boundary-Value Problem for a Second-Order Hyperbolic Equation. II. Quasilinear Problem”.
Ukrains’kyi Matematychnyi Zhurnal
50 (12): 1680–1685.
https://umj.imath.kiev.ua/index.php/umj/article/view/4787
.