NIZHNIK, L. P.; ALBEVERIO, S. Schrödinger operators with a number of negative eigenvalues equal to the number of point interactions. Methods of Functional Analysis and Topology, [S. l.], v. 9, n. 4, p. 273–286, 2003. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/222. Acesso em: 4 feb. 2026.