3605

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


 * $5 \times 7 \times 103$


 * The $35$th octagonal number, after $1$, $8$, $21$, $40$, $65$, $\ldots$, $2296$, $2465$, $2640$, $2821$, $3008$, $3201$, $3400$:
 * $3605 = \displaystyle \sum_{k \mathop = 1}^{35} \left({6 k - 5}\right) = 35 \left({3 \times 35 - 2}\right)$