WALLIN, N. O. A Problem for Elliptic Partial Differential Operators Defined on an Arbitrary Open Set. MATHEMATICA SCANDINAVICA, [S. l.], v. 9, p. 185–196, 1961. DOI: 10.7146/math.scand.a-10634. Disponível em: https://www.mscand.dk/article/view/10634. Acesso em: 5 may. 2024.