Fermat number

A number of the form . In binary notation, a Fermat number is 2^n-1 zeroes between an initial and final 1. The first Fermat number is = 3, or 11 in binary.

In 1999, Richard E. Crandall, Ernst W. Mayer, and Jason S. Papadopoulos proved that the 24th Fermat number, , which has more than five million digits, is not a prime. Their computation required more than 10¹⁷ computer operations.