51

Number
$51$ (fifty-one) is:


 * $3 \times 17$


 * The $9$th trimorphic number after $1, 4, 5, 6, 9, 24, 25, 49$:
 * $51^3 = 132 \, 6 \mathbf {51}$


 * The $6$th pentagonal number after $1, 5, 12, 22, 35$:
 * $51 = 1 + 4 + 7 + 10 + 13 + 16 = \dfrac {6 \left({3 \times 6 - 1}\right)} 2$


 * The $18$th semiprime after $4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49$:
 * $51 = 3 \times 17$


 * The $14$th lucky number:
 * $1, 3, 7, 9, 13, 15, 21, 25, 31, 33, 37, 43, 49, 51, \ldots$


 * The $3$rd term of the $1$st $5$-tuple of consecutive integers have the property that they are not values of the $\sigma$ function $\sigma \left({n}\right)$ for any $n$:
 * $\left({49, 50, 51, 52, 53}\right)$


 * The $23$rd after $1, 2, 4, 5, 6, 8, 9, 12, 13, 15, 16, 17, 20, 24, 25, 27, 28, 32, 35, 26, 39, 48$ of the $24$ positive integers which cannot be expressed as the sum of distinct non-pythagorean primes.


 * Suggested by in his $1997$ book  as being the smallest uninteresting number, which fact makes it intrinsically interesting.

Also see

 * Interesting Number Paradox