GUSTAFSSON, B.; SAKAI, Makoto. An Approximation Theorem for Integrable Harmonic Vector Fields. Mathematica Scandinavica, Berkeley, CA, United States, v. 70, p. 78–90, 1992. DOI: 10.7146/math.scand.a-12387. Disponível em: https://journals.msp.org/mscand/article/view/1257. Acesso em: 24 jul. 2026.