Maps use colors to distinguish one region from another by ensuring that adjacent regions are colored with different colors. In 1852 Francis Guthrie noticed that a map of the counties of England could be colored using only four countries. The claim that four countries suffice to color any planar map is called the four-color theorem and was only proved in 1976 by Kenneth Appel andWolfgang Haken. They used sophisticated mathematical arguments to show that if there is a counterexample (a map needing more than four colors), it had to be associated with one of 1834 configurations. They then used a computer to check these configurations.
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Ben-Ari, M. (2022). The Five-Color Theorem. In: Mathematical Surprises. Springer, Cham. https://doi.org/10.1007/978-3-031-13566-8_4
Publisher Name: Springer, Cham
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