BURYACHENKO, E. A. Conditions of nontrivial solvability of the homogeneous Dirichlet problem for equations of any even order in the case of multiple characteristics without slope angles. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 62, n. 5, p. 591–603, 2010. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2890. Acesso em: 27 apr. 2025.