ASPLUND, E.; GRÜNBAUM, B. On a Coloring Problem. Mathematica Scandinavica, Berkeley, CA, United States, v. 8, p. 181–188, 1960. DOI: 10.7146/math.scand.a-10607. Disponível em: https://journals.msp.org/mscand/article/view/2738. Acesso em: 25 jul. 2026.