HASSI, S.; DE SNOO, H. One-dimensional graph perturbations of selfadjoint relations. Annales Fennici Mathematici, [S. l.], v. 22, n. 1, p. 123–164, 1997. Disponível em: https://afm.journal.fi/article/view/134884. Acesso em: 18 maj. 2024.