MITROPOLSKIY, Y. A.; KHOMA-MOHYL’S’KA, S. H. 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, v. 57, n. 7, p. 912–921, 25 Jul. 2005.