*Map created by Fibonacci on Wikimedia*

The map shows the four colour theorem in practice.

The theorm states that:

… given any separation of a plane into contiguous regions, producing a figure called a map, no more than four colors are required to color the regions of the map so that no two adjacent regions have the same color. Adjacent means that two regions share a common boundary curve segment, not merely a corner where three or more regions meet.

In actual, fact the map uses 5 colours, if you include the white used for the oceans, although it would still technically be possible to draw a world map using only 4 colours, if landlocked countries shared the same colour as the ocean.

Want to read more on the subject? The good news is that there’s a surprising number of books about it:

- Four Colors Suffice: How the Map Problem Was Solved
- The Four-Color Theorem: History, Topological Foundations, and Idea of Proof
- The Four Colour Theorem
- Graphs, Colourings and the Four-Colour Theorem

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