Azizov, M. “On One Direct Method for the Approximate Solution of a Periodic Boundary-Value Problem”. Ukrains’kyi Matematychnyi Zhurnal 49, no. 9 (September 25, 1997): 1157–1161. Accessed May 17, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5112.