641

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


 * The $116$th prime number


 * The $1$st of the $29$th pair of twin primes:
 * $643 - 641 = 2$


 * The $28$th Sophie Germain prime after $2$, $3$, $5$, $11$, $23$, $29$, $41$, $\ldots$, $233$, $239$, $251$, $281$, $293$, $359$, $419$, $431$, $443$, $491$, $509$, $593$:
 * $2 \times 641 + 1 = 1283$, which is prime.


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