2640

Number
$2640$ (two thousand, six hundred and forty) is:


 * $2^4 \times 3 \times 5 \times 11$


 * The $30$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $1281$, $1408$, $1541$, $1680$, $1825$, $1976$, $2133$, $2296$, $2465$:
 * $2640 = \displaystyle \sum_{k \mathop = 1}^{30} \left({6 k - 5}\right) = 30 \left({3 \times 30 - 2}\right)$