Dr Gonthier used similar techniques to those of Dr Appel and Dr Haken in his proof.

ECONOMIST: Just what does it mean to prove something?

The star informer was Trevor Haken, a former detective sergeant who helped to install miniature video cameras in the dashboard of a police car and in the light fittings of a prostitute's home.

ECONOMIST: Australia

It was first proposed in 1852 but, despite efforts by a century's worth of mathematicians, went unproven until 1976, when Kenneth Appel and Wolfgang Haken, then of the University of Illinois, announced that they had proved the result.

ECONOMIST: Just what does it mean to prove something?