DERKACH, V. A.; HASSI, S.; DE SNOO, H. Operator models associated with singular perturbations. Methods of Functional Analysis and Topology, [S. l.], v. 7, n. 3, p. 1–21, 2001. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/154. Acesso em: 19 dec. 2025.