2640

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Number

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

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

The $30$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $1281$, $1408$, $1541$, $1680$, $1825$, $1976$, $2133$, $2296$, $2465$:

$2640 = \ds \sum_{k \mathop = 1}^{30} \paren {6 k - 5} = 30 \paren {3 \times 30 - 2}$

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