Swart, E. R. (1980). «The Philosophical Implications of the Four-Color Problem». The American Mathematical Monthly87 (9): 697-702. ISSN0002-9890. JSTOR2321855. doi:10.2307/2321855.
Swart, E. R. (1980). «The Philosophical Implications of the Four-Color Problem». The American Mathematical Monthly87 (9): 697-702. ISSN0002-9890. JSTOR2321855. doi:10.2307/2321855.
jstor.org
Swart, E. R. (1980). «The Philosophical Implications of the Four-Color Problem». The American Mathematical Monthly87 (9): 697-702. ISSN0002-9890. JSTOR2321855. doi:10.2307/2321855.
Wilson, Robin (2014). Four colors suffice : how the map problem was solved (Revised color edición). Princeton, New Jersey: Princeton University Press. pp. 216-222. ISBN9780691158228. OCLC847985591.