GUSTAFSSON, B.; SAKAI, M. An Approximation Theorem for Integrable Harmonic Vector Fields. MATHEMATICA SCANDINAVICA, [S. l.], v. 70, p. 78–90, 1992. DOI: 10.7146/math.scand.a-12387. Disponível em: https://www.mscand.dk/article/view/12387. Acesso em: 30 apr. 2024.