GORBACHUK, V. I.; GORBACHUK, M. L. Some problems of spectral theory of the second order linear differential equation with unlimited operator coefficients . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 23, n. 1, p. 3–14, 1970. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8473. Acesso em: 6 dec. 2025.