Pierre de Fermat (1601 – 1665) was a French lawyer and arguably the greatest amateur mathematician that ever lived.
He owned a copy of Diophantus’ Arithmetica and set about proving results he found in that book. It was not unusual for him to spend three or four days and nights working solidly on a proof and then just to disregard the work. Often, the only record we have of his efforts is his notes in the margin.
Mathematicians have set about trying to reproduce his results, and the most famous and elusive result became known as the ‘Last Theorem’ and took around three hundred and fifty years to solve. It states that no three positive integers a, b, and c can satisfy the equation an + bn = cn for any integer value of n greater than two. He claimed to have found a proof, but could not fit it into the margin!
The result was finally proven in 1995 by the Princeton mathematician Andrew Wiles after seven years’ dedicated work, and in the process he revolutionised algebraic number theory.
Fermat is my mathematical hero because of his pursuit of knowlegde purely for its own sake and his sheer genius.