3605

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Number

$3605$ (three thousand, six hundred and five) is:

$5 \times 7 \times 103$

The $35$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $2296$, $2465$, $2640$, $2821$, $3008$, $3201$, $3400$:

$3605 = \ds \sum_{k \mathop = 1}^{35} \paren {6 k - 5} = 35 \paren {3 \times 35 - 2}$

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