WALLIN, N. O. A Problem for Elliptic Partial Differential Operators Defined on an Arbitrary Open Set. Mathematica Scandinavica, Berkeley, CA, United States, v. 9, p. 185–196, 1961. DOI: 10.7146/math.scand.a-10634. Disponível em: https://journals.msp.org/mscand/article/view/2710. Acesso em: 25 jul. 2026.