1825

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Number

$1825$ (one thousand, eight hundred and twenty-five) is:

$5^2 \times 73$

The $25$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $645$, $736$, $833$, $936$, $1045$, $1160$, $1281$, $1408$, $1541$, $1680$:

$1825 = \ds \sum_{k \mathop = 1}^{25} \paren {6 k - 5} = 25 \paren {3 \times 25 - 2}$

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