BURSKII, V. P. Conditions of regularity of a general differential boundary-value problem for improperly elliptic equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 62, n. 6, p. 754–761, 2010. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2906. Acesso em: 7 apr. 2025.