Fermat's Last Theorem
Fermat is a famous mathematician even today - the inventor of differential calculus, the finest number theorist between Archimedes and Gauss, and, in addition, a full-time lawyer.
HISTORY OF FERMAT'S LAST THEOREM
For over 400 years, Fermat's Last Theorem was merely a conjecture. Formulated in the year +/- 1650 by a French mathematician, the "theorem" stood out for it's simplicity: it is a simple extension of the famous Theorem of Pythagoras, which we have all heard about. Fermat's Last Theorem is a statement, that we will never have an extension of the Theorem of Pythagoras for integer exponents are greater than 2. In other words, though there were many triples of numbers that "solved" the famous equation (x)^2 + (y)^2 = (z)^2, there would be no such integer triples to solve the similar equation (x)^3 +(y)^3 = (z)^3, nor indeed ever, for any integer exponent greater than 2.
WAITING FOR THE PROOF
Simple-sounding though it may be, the "etxtened" Theorem of Pythagoras was proved... after about 400 years. Imagine that: four hundred years, during which not one person ever found any example of three numbers that fit that equation, for any exponent greater than 2.
I never forget the time spent by those tireless and for the most part nameless mathematicians who spent untold and undocumented hours, trying to solve Fermat's Last Theorem - and failing. Eventually, after 400 years, their cumulative efforts were rewarded, when one of their number offered what is generally regarded as proof - 89 pages' worth.
Even today, Fermat's story retains the flavour of mystery. Students of the topic know, that Fermat famously suggested that an elegant proof of the theorem did exist, but which was too long to fit in the margin of the book in which the theorem was stated! That proof, if it exists, is still undiscovered.