521/Mistake

Source Work

 * The Dictionary
 * $2^{521} - 1$
 * $2^{521} - 1$

Mistake

 * In a few hours on the night of $30$ January $1952$, using the SWAC computer, proved that $2^{521} - 1$ and the $183$-digit number $2^{607} - 1$ are both Mersenne primes.

This statement goes against the usual account that it was who (by writing and running a computer program) accomplished this feat.

However, it is worth pointing out that the algorithm he implemented was designed by in the first place.