PURMONEN, V. T. On parametrically quasi-elliptic boundary problems. Annales Fennici Mathematici, [S. l.], v. 5, n. 2, p. 237–262, 1980. DOI: 10.5186/aasfm.1980.0521. Disponível em: https://afm.journal.fi/article/view/134385. Acesso em: 25 nov. 2024.