Fermat's Last Theorem
This means that there are no examples where ,  and  are natural numbers, i.e. whole numbers larger than zero, and where  is a whole number bigger than 2. Pierre de Fermat wrote about it in 1637 inside his copy of a book called Arithmetica. He said "I have a proof of this theorem, but there is not enough space in this margin". However, no correct proof was found for 357 years. It was finally proven in 1995. Most mathematicians do not think that Fermat, in fact, ever had a margin proof of this theorem.