Episode 2 of 2

Observation of Four Colors

When: 23 October 1852
Where: unspecified
Part of: Francis Guthrie conjectures the four color theorem at University College London

Francis Guthrie noticed that only four colors were needed to color the map of counties of England, leading to the conjecture.

On October 23, 1852, Francis Guthrie was trying to color the map of counties of England. He observed that only four different colors were sufficient to ensure that no two adjacent counties shared the same color. This specific observation led to the formulation of the four color conjecture.

Sources

Four color theorem Wikipedia, revision of 2026-09-20

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