FABRICIUS-BJERRE, Fr. A relation between the numbers of singular points and singular lines of a plane closed curve. Mathematica Scandinavica, Berkeley, CA, United States, v. 40, p. 20–24, 1977. DOI: 10.7146/math.scand.a-11672. Disponível em: https://journals.msp.org/mscand/article/view/1974. Acesso em: 25 jul. 2026.