1281

Number
$1281$ (one thousand, two hundred and eighty-one) is:


 * $3 \times 7 \times 61$


 * The $21$st octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $645$, $736$, $833$, $936$, $1045$, $1160$:
 * $1281 = \displaystyle \sum_{k \mathop = 1}^{21} \left({6 k - 5}\right) = 21 \left({3 \times 21 - 2}\right)$