**On 19 April 2013, a pioneer of mathematics passed away. Kenneth Appel’s most famous work involved a new and controversial way of doing mathematics – using a computer.**

Mathematics is based on proofs. A proof is a train of thought showing something is definitely true – or definitely false. When you read a proof and understand it, you’ll know the author’s reasoning, and see why they are correct.

In 1976, Kenneth and his colleague Wolfgang Haken came up with a way of proving a famous problem by breaking it into lots of smaller examples. There were too many examples for them to check by hand, so they got a computer to do the checking.

Many mathematicians didn’t like this new way of doing maths. They couldn’t read the whole proof because it was much too long. They didn’t trust a computer to do the checking. Maybe there was a mistake in the program, or maybe the computer made a mistake – no one could check all the working to make sure it was correct.

Since Kenneth and Wolfgang’s proof, there have been many more computer assisted proofs for different problems. They are still controversial, but they are becoming more accepted. This pioneering work has helped mathematicians utilise the power of computers.