This is Fermats last theorum. It's easy to understand but resisted the best minds for 350 years. Consequently maths professors were forever getting letters from crazies saying they'd proved it.
Pythagorus's theorom says the square of the long side of a triangle is equal to the sum of the square of the other two sides - There are whole numbers that this works for e.g
3,4,5 (3 squared -9, plus 4 squared -16 = 5 squared -25)
Fermat said that this only works for squares that there are no sets of integers a,b,c where say a cubed +b cubed = c cubed - or to the fourth power or any orther power only 2.
This was finally proved (to the relief of maths professors) in 1994 and required very state of the art maths.
So basically Fermat was bluffing
http://en.wikipedia.org/wiki/Fermat's_last_the orem