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Fabian Franklin. William Edward Story (1850-1930). Proceedings of the American Academy of Arts and Sciences. 1936年3月, 70 (10): 578–580 [2013-03-01]. (原始内容存档于2016-08-19) (英语).
Gerhard Ringel, J.W.T.Youngs. Solution of the Heawood Map-coloring Problem(PDF). Proceedings of the National Academy of Science of the U.S.A. 1968, 60 (2): 438–445 [2013-03-11]. (原始内容存档(PDF)于2021-11-08) (英语).
Fabian Franklin. William Edward Story (1850-1930). Proceedings of the American Academy of Arts and Sciences. 1936年3月, 70 (10): 578–580 [2013-03-01]. (原始内容存档于2016-08-19) (英语).
Professors Ralph Bravaco and Shai Simonson. The Four-Color Theorem. Stonehill College. [2013-03-02]. (原始内容存档于2013-05-01) (英语).
Robin Thomas. An Update on the Four-Color Theorem(PDF). Notices of American Mathematics Society. 1998, 45: 848–859 [2013-03-03]. (原始内容存档(PDF)于2010-03-31) (英语).
J. J. O'Connor, E. F. Robertson. The four colour theorem. 犹他州大学. 1996年9月 [2013-03-03]. (原始内容存档于2013-01-16) (英语).
Gerhard Ringel, J.W.T.Youngs. Solution of the Heawood Map-coloring Problem(PDF). Proceedings of the National Academy of Science of the U.S.A. 1968, 60 (2): 438–445 [2013-03-11]. (原始内容存档(PDF)于2021-11-08) (英语).