FRIBERG, J. Partially Hypoelliptic Differential Equations of Finite Type. MATHEMATICA SCANDINAVICA, [S. l.], v. 9, p. 22–42, 1961. DOI: 10.7146/math.scand.a-10620. Disponível em: https://www.mscand.dk/article/view/10620. Acesso em: 3 jul. 2024.