DOPPEL, K.; HOCHMUTH, R. On the regularity of solutions of a homogeneous Dirichlet problem for a non-hypoelliptic linear partial differential operator. Annales Fennici Mathematici, [S. l.], v. 16, n. 1, p. 183–198, 1991. DOI: 10.5186/aasfm.1991.1606. Disponível em: https://afm.journal.fi/article/view/134715. Acesso em: 25 nov. 2024.