KATS, I. S. Spectral theory of a string. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 3, p. 155–176, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5754. Acesso em: 6 dec. 2025.