9591

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Number

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

$3 \times 23 \times 139$

The $138$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $8911$, $9045$, $9180$, $9316$, $9453$:

$9591 = \ds \sum_{k \mathop = 1}^{138} k = \dfrac {138 \times \paren {138 + 1} } 2$

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