This means that there are no examples where formula_3, formula_4 and formula_5 are natural numbers, i.e. whole numbers larger than zero, and where formula_1 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. Mathematicians everywhere think that Fermat, in fact, did not have a good proof of this theorem.