BASKAKOV, A. G.; CHERNYSHOV, M. K. On the invertibility of differential operators of the second order. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 3, p. 411–413, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5432. Acesso em: 14 apr. 2025.