Swart, E. R. (1980). "The Philosophical Implications of the Four-Color Problem". The American Mathematical Monthly. 87 (9): 697–702. doi:10.2307/2321855. ISSN0002-9890. JSTOR2321855.
Swart, E. R. (1980). "The Philosophical Implications of the Four-Color Problem". The American Mathematical Monthly. 87 (9): 697–702. doi:10.2307/2321855. ISSN0002-9890. JSTOR2321855.
Swart, E. R. (1980). "The Philosophical Implications of the Four-Color Problem". The American Mathematical Monthly. 87 (9): 697–702. doi:10.2307/2321855. ISSN0002-9890. JSTOR2321855.
Wilson, Robin (2014). Four colors suffice : how the map problem was solved (Revised color ed.). Princeton, New Jersey: Princeton University Press. pp. 216–222. ISBN9780691158228. OCLC847985591.