LYUBICH, Y. Sturm-Liouville problem with a distributed condition. Математическая физика, анализ, геометрия, Харків, Україна, v. 10, n. 3, p. 290–300, 2003. Disponível em: https://mag.ilt.kharkiv.ua/index.php/mag/article/view/m10-0290e. Acesso em: 4 груд. 2024.