OGURISU, O. On the number of negative eigenvalues of a Schrodinger operator with $\delta$ interactions. Methods of Functional Analysis and Topology, [S. l.], v. 16, n. 1, p. 42–50, 2010. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/438. Acesso em: 16 dec. 2025.