BorokV. M., and KengneE. “Classification of Nonlocal Boundary-Value Problems on a Narrow Strip”. Ukrains’kyi Matematychnyi Zhurnal 46, no. 4 (April 25, 1994): 338–346. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5730.