Pierre de Fermat (1601–65) was a French lawyer and amateur mathematician. He first conjectured his "last theorem" in 1637, claiming that he had a proof; but he never wrote his proof down, and the theorem was famously not proved until 1995. Prior to this, it had been listed in the Guinness Book of Records as "the most difficult maths problem".
Fermat stated his theorem thus: "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." In other words: there is no cube that's the sum of two smaller cubes, no fourth power that's the sum of two smaller fourth powers, and so on.
© Haydn Thompson 2017