Staněk S. Existence principles for higher-order nonlocal boundary-value problems and their applications to singular Sturm-Liouville problems. Ukr. Mat. Zhurn. [Internet]. 2008Feb.25 [cited 2024Apr.19];60(2):240–259. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/3153