FABRICIUS-BJERRE, Fr. A relation between the numbers of singular points and singular lines of a plane closed curve. MATHEMATICA SCANDINAVICA, [S. l.], v. 40, p. 20–24, 1977. DOI: 10.7146/math.scand.a-11672. Disponível em: https://tidsskrift.dk/math/article/view/11672. Acesso em: 14 aug. 2026.