1071

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


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