Today's Google logo pays tribute to a 17th Century math genius, Pierre de Fermat, and his Last Theorem, which took centuries to solve. Though he was trained and worked as a lawyer, he ranked with Rene Descartes as a numbers theorist.

But he is remembered for a work undone, as this Christian Science Monitor article explains:

"One day in 1637, while perusing his copy of an ancient Greek text by the 3rd century mathematician Diophantus, Fermat jotted a note in the margins that would drive mathematicians crazy for the next four centuries. Fermat's marginalia, which was written in Latin and later discovered by his son after he died, read: 'It is impossible to separate a cube into two cubes, or a fourth power into two fourth powers, or in general, any power higher than the second, into two like powers. I have discovered a truly marvelous proof of this, which this margin is too narrow to contain.' In other words, a to the nth + b to the nth can never equal c to the nth, as long as a, b, and c are positive integers and as long as n is greater than two."

The solution was finally discovered in the 1990s by Andrew Wiles of Princeton University -- a tale recounted in "Fermat's Enigma: The Epic Quest to Solve the World's Greatest Mathematical Problem" by Simon Singh.