STANĚK, S. Existence principles for higher-order nonlocal boundary-value problems and their applications to singular Sturm-Liouville problems. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 60, n. 2, p. 240–259, 2008. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3153. Acesso em: 6 dec. 2025.