The Four Colour Theorem

The Four Colour Theorem states that any map can be coloured with at most four colours so that no two adjacent regions share a colour. Francis Guthrie conjectured this in 1852 while colouring a map of England. It took 124 years to prove. In 1976, Kenneth Appel and Wolfgang Haken proved it using a computer to check 1,936 cases — the first major theorem proved by computer. Some mathematicians were troubled: was it a real proof if no human could verify every step?