HANSEN, Vagn Lundsgaard. On the Space of Maps of a Closed Surface into the 2-Sphere. Mathematica Scandinavica, Berkeley, CA, United States, v. 35, p. 149–158, 1974. DOI: 10.7146/math.scand.a-11542. Disponível em: https://journals.msp.org/mscand/article/view/2099. Acesso em: 25 jul. 2026.