89

Number
$89$ (eighty-nine) is:


 * The $24$th prime number


 * The $10$th Sophie Germain prime after $2, 3, 5, 11, 23, 29, 41, 53, 83$:
 * $2 \times 89 + 1 = 179$, which is prime.


 * The $11$th Fibonacci number, after $1, 1, 2, 3, 5, 8, 13, 21, 34, 55$:
 * $89 = 34 + 55$


 * The $5$th Fibonacci prime after $2, 3, 5, 13$.


 * The $6$th of the sequence of $n$ such that $p_n \# - 1$, where $p_n \#$ denotes primorial of $n$, is prime:
 * $3, 5, 11, 13, 41, 89$


 * The smaller of the smallest pair of primes whose prime gap is $8$:
 * $97 - 89 = 8$


 * The $1$st term of the smallest sequence of $6$ primes formed by doubling and adding one:
 * $\left({89, 179, 359, 719, 1439, 2879}\right)$


 * The $11$th number after $0, 1, 2, 3, 4, 5, 6, 7, 8, 9$ which is the sum of the increasing powers of its digits taken in order:
 * $8^1 + 9^2 = 89$


 * The $45$th positive integer after $2, 3, 4, 7, 8, \ldots, 61, 65, 66, 67, 72, 77, 80, 81, 84$ which cannot be expressed as the sum of distinct pentagonal numbers.


 * With $98$, gives the longest reverse-and-add sequence of any $2$-digit integers, of $24$ terms.

Also see

 * 2-Digit Numbers forming Longest Reverse-and-Add Sequence