When Pierre de Fermat conjectured his last theorem, of which what would be known as the Fermat’s Last Theorem, in 1637, he only wrote it on the margin of his copy of Arithmetica. He claimed that he knew the proof, but it was too large to fit in the narrow margin.
H. H. Wu, mathematician and professor emeritus at Berkeley, related that he saw the following writing on the wall of a subway station in New York City:
I have just found a truly marvelous proof of Fermat’s Last Theorem. But the train is coming and I don’t have time to write it down.