1281

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Number

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

$3 \times 7 \times 61$

The $21$st octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $645$, $736$, $833$, $936$, $1045$, $1160$:

$1281 = \ds \sum_{k \mathop = 1}^{21} \paren {6 k - 5} = 21 \paren {3 \times 21 - 2}$

Also see

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