8640

Number
$8640$ (eight thousand, six hundred and forty) is:


 * $2^6 \times 3^3 \times 5$


 * The $54$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $5985$, $6256$, $6533$, $6816$, $7105$, $7400$, $7701$, $8008$, $8321$:
 * $8640 = \displaystyle \sum_{k \mathop = 1}^{54} \left({6 k - 5}\right) = 54 \left({3 \times 54 - 2}\right)$