Čiegis, R., A. Štikonas, O. Štikoniene, and O. Suboč. “Stationary Problems With Nonlocal Boundary Conditions”. Mathematical Modelling and Analysis 6, no. 2 (December 15, 2001): 178-191. Accessed September 20, 2021. https://journals.vgtu.lt/index.php/MMA/article/view/9897.