S. Staněk, “Existence principles for higher-order nonlocal boundary-value problems and their applications to singular Sturm-Liouville problems”, Ukr. Mat. Zhurn., vol. 60, no. 2, pp. 240–259, Feb. 2008, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3153