1071
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Number
$1071$ (one thousand and seventy-one) is:
- $3^2 \times 7 \times 17$
- The $21$st heptagonal number after $1$, $7$, $18$, $34$, $55$, $81$, $112$, $148$, $189$, $235$, $286$, $342$, $403$, $469$, $540$, $616$, $697$, $783$, $874$, $970$:
- $1071 = \ds \sum_{k \mathop = 1}^{21} \paren {5 k - 4} = \dfrac {21 \paren {5 \times 21 - 3} } 2$