NIZHNIK, L. P. Spectral analysis of metric graphs with infinite rays. Methods of Functional Analysis and Topology, [S. l.], v. 20, n. 4, p. 391–396, 2014. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/594. Acesso em: 17 dec. 2025.