641

Number
$641$ (six hundred and forty-one) is:


 * The $116$th prime number


 * One of the two prime factors of the $6$th Fermat number $2^{\left({2^5}\right)} + 1$, the other being $6 \, 700 \, 417$:
 * $641 = 5 \times 2^7 + 1$