ÖzenK. “Green’s Functional for Higher-Order Ordinary Differential Equations With General nonlocal conditions and Variable Principal Coefficient”. Ukrains’kyi Matematychnyi Zhurnal 71, no. 1 (January 25, 2019): 99-116. Accessed December 5, 2020. http://umj.imath.kiev.ua/index.php/umj/article/view/1422.