1001

Number
$1001$ (one thousand and one) is:


 * $7 \times 11 \times 13$


 * The $26$th pentagonal number after $1$, $5$, $12$, $22$, $35$, $\ldots$, $477$, $532$, $590$, $651$, $715$, $782$, $852$, $925$:
 * $1001 = \displaystyle \sum_{k \mathop = 1}^{26} \left({3 k - 2}\right) = \dfrac {26 \left({3 \times 26 - 1}\right)} 2$