CARLSSON, A. M. V. On Approximation in Weighted Sobolev Spaces and Self-Adjointness. MATHEMATICA SCANDINAVICA, [S. l.], v. 74, p. 111–124, 1994. DOI: 10.7146/math.scand.a-12483. Disponível em: https://www.mscand.dk/article/view/12483. Acesso em: 5 may. 2024.