IZVEKOV, I. G.; MARTYNENKO, E. V. On Gevrais classes of certain self-adjoint differential operators with degeneration. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 12, p. 1622–1626, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5969. Acesso em: 11 apr. 2025.