Petrovskii, Ya. B., and G. P. Khoma. “The Solvability of a Boundary-Value Periodic Problem”. Ukrains’kyi Matematychnyi Zhurnal 49, no. 2 (February 25, 1997): 302–308. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5006.