1771

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Number

$1771$ (one thousand, seven hundred and seventy-one) is:

$7 \times 11 \times 23$

The $21$st tetrahedral number, after $1$, $4$, $10$, $20$, $35$, $\ldots$, $816$, $969$, $1140$, $1330$, $1540$:

$1771 = \ds \sum_{k \mathop = 1}^{21} \frac {k \paren {k + 1} } 2 = \dfrac {21 \paren {21 + 1} \paren {21 + 2} } 6$

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