991

Number
$991$ (nine hundred and ninety-one) is:


 * The $167$th prime number


 * The $9$th prime $p$ such that $p \# - 1$, where $p \#$ denotes primorial (product of all primes up to $p$) of $p$, is prime:
 * $3$, $5$, $11$, $13$, $41$, $89$, $317$, $337$, $991$


 * The larger of the $15$th pair of primes whose prime gap is $8$:
 * $991 - 983 = 8$


 * The $22$nd permutable prime after $2$, $3$, $5$, $7$, $11$, $13$, $17$, $31$, $37$, $71$, $73$, $79$, $97$, $113$, $131$, $199$, $311$, $337$, $373$, $733$, $919$


 * The $33$rd emirp after $13$, $17$, $31$, $37$, $\ldots$, $733$, $739$, $743$, $751$, $761$, $769$, $907$, $937$, $941$, $953$, $967$, $971$, $983$