Four Colours Suffice: How the Map Problem Was Solved - Tapa dura

The problem is simply stated, and involves the colouring of maps. Can every map be coloured with at most four colours so that neighbouring countries are coloured differently?

First posed 150 years ago, it took more than a century of colouring maps and developing the necessary theoretical machinery before the result was established with certainty. Even then, difficult philosophical questions remained.

With elegance and clarity, Robin Wilson explains the problem and the proof.