TUHAI, H. V. On new points of the discrete spectrum under singular perturbations. Methods of Functional Analysis and Topology, [S. l.], v. 24, n. 3, p. 288–296, 2018. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/702. Acesso em: 15 dec. 2025.