ARLINSKII, Yu. M.; TSEKANOVSKII, E. Some remarks on singular perturbations of self-adjoint operators. Methods of Functional Analysis and Topology, [S. l.], v. 9, n. 4, p. 287–308, 2003. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/223. Acesso em: 17 dec. 2025.