1680

Number
$1680$ (one thousand, six hundred and eighty) is:


 * $2^4 \times 3 \times 5 \times 7$


 * The $24$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $645$, $736$, $833$, $936$, $1045$, $1160$, $1281$, $1408$, $1541$:
 * $1680 = \displaystyle \sum_{k \mathop = 1}^{24} \left({6 k - 5}\right) = 24 \left({3 \times 24 - 2}\right)$