MitropolskiyYu. A., and Khoma-Mohyl’s’kaS. H. 2005. “Conditions for the Existence of Solutions of a Periodic Boundary-Value Problem for an Inhomogeneous Linear Hyperbolic Equation of the Second Order. I”. Ukrains’kyi Matematychnyi Zhurnal 57 (7), 912–921. https://umj.imath.kiev.ua/index.php/umj/article/view/3653.