In 1993, banker and amateur mathematician Andrew Beal proposed that if A^x + B^y = C^z, where A, B, C, x, y, and z are positive integers and x, y, and z are all greater than 2, then A, B, and C must have a common prime factor.

Is it true? No one knows, but Beal is offering $1 million for a peer-reviewed proof or a counterexample.