PURMONEN, V. T. On generalized a priori estimates for quasi-elliptic differential operators. Annales Fennici Mathematici, [S. l.], v. 4, n. 2, p. 227–252, 1979. DOI: 10.5186/aasfm.1978-79.0420. Disponível em: https://afm.journal.fi/article/view/134351. Acesso em: 25 nov. 2024.