9591

Number
$9591$ (nine thousand, five hundred and ninety-one) is:


 * $3 \times 23 \times 139$


 * The $138$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $8911$, $9045$, $9180$, $9316$, $9453$:
 * $9591 = \displaystyle \sum_{k \mathop = 1}^{138} k = \dfrac {138 \times \left({138 + 1}\right)} 2$