PEETRE, Jaak. On Estimating the Solutions of Hypoelliptic Differential Equations Near the Plane Boundary. Mathematica Scandinavica, Berkeley, CA, United States, v. 9, p. 337–351, 1961. DOI: 10.7146/math.scand.a-10639. Disponível em: https://journals.msp.org/mscand/article/view/2705. Acesso em: 8 sep. 2026.