TVERBERG, H. On a four-colour theorem. MATHEMATICA SCANDINAVICA, [S. l.], v. 39, p. 267–270, 1976. DOI: 10.7146/math.scand.a-11661. Disponível em: https://www.mscand.dk/article/view/11661. Acesso em: 6 may. 2024.