PARAMONOV, P. V.; VERDERA, J. Approximation by Solutions of Elliptic Equations on Closed Subsets of Euclidean Space. Mathematica Scandinavica, Berkeley, CA, United States, v. 74, p. 249–259, 1994. DOI: 10.7146/math.scand.a-12493. Disponível em: https://journals.msp.org/mscand/article/view/1132. Acesso em: 25 jul. 2026.