51

Number
$51$ (fifty-one) is:


 * $3 \times 17$


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