SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Four colour problem. Map of the United States of America (USA) being used to illustrate the four colour problem. This is a famous mathematical theorem that says that it is possible to colour different regions of a map (or any similar surface) using only four colours, while still ensuring that no adjacent regions are the same colour. This theorem was first proposed as a conjecture in 1862, and was proven with the aid of a computer in 2004.
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