VALQUI CASAS, Holger. El problema de los cuatro colores. Pro Mathematica, [S. l.], v. 8, n. 15-16, p. 47–57, 1994. Disponível em: https://revistas.pucp.edu.pe/index.php/promathematica/article/view/10175. Acesso em: 24 dic. 2025.