Khoma, N. H., and L. G. Khoma. “A Linear Periodic Boundary-Value Problem for a Second-Order Hyperbolic Equation”. Ukrains’kyi Matematychnyi Zhurnal, vol. 51, no. 2, Feb. 1999, pp. 281–284, https://umj.imath.kiev.ua/index.php/umj/article/view/4611.