Prime Decomposition of 5th Fermat Number/Mistake

Source Work

 * The Dictionary
 * $641$
 * $641$

Mistake

 *  found the first counterexample to conjecture that $2^{2^n} + 1$ is always prime, when he discovered in $1742$ that $2^{2^5} + 1$ is divisible by $641$.

The date was in fact $1732$.

In the entry for $257$, gets the date right.