Four Color Theorem

Every planar map needs at most four colours so that no two touching regions share one.

Specifications & Metrics

proved

Appel and Haken, 1976 — by computer

Core Principles & Analysis

• Francis Guthrie posed it in 1852 colouring a map of England's counties.
• Its 1976 proof checked 1,936 configurations by computer — the first major proof no human can read.
• Philosophers of mathematics still argue whether a proof nobody can check by hand is a proof.