1071

Number
$1071$ (one thousand and seventy-one) is:
 * $3^2 \times 7 \times 17$


 * The $21$st heptagonal number after $1$, $7$, $18$, $34$, $55$, $81$, $112$, $148$, $189$, $235$, $286$, $342$, $403$, $469$, $540$, $616$, $697$, $783$, $874$, $970$:
 * $1071 = \displaystyle \sum_{k \mathop = 1}^{21} \left({5 k - 4}\right) = \dfrac {21 \left({5 \times 21 - 3}\right)} 2$