What was the longest-standing incorrect proof of the Four Color Theorem in graph theory?
The longest-standing incorrect proof of the Four Color Theorem in graph theory was produced by Kempe in 1879. He claimed to have proved the theorem using a technique known as “reduction to absurdity”, which involved coloring maps and deriving a contradiction when certain conditions were not met. However, this proof was later discovered to be flawed and was soon corrected by other mathematicians. It took over a century of work by many mathematicians to finally prove the Four Color Theorem rigorously, with the first correct proof being published in 1976.
What is the Four Colour Theorem and why was Kempe's proof flawed?