TYSHKEVICH, D. L.; KARPENKO, I. I. On self-adjontness of 1-D Schrödinger operators with $\delta$-interactions. Methods of Functional Analysis and Topology, [S. l.], v. 18, n. 4, p. 360–372, 2012. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/527. Acesso em: 16 dec. 2025.