861

Number
$861$ (eight hundred and sixty-one) is:


 * $3 \times 7 \times 41$


 * The $21$st hexagonal number after $1, 6, 15, 28, 45, 66, 91, \ldots, 378, 435, 496, 561, 630, 703, 780$:
 * $861 = \displaystyle \sum_{k \mathop = 1}^{21} \left({4 k - 3}\right) = 21 \left({2 \times 21 - 1}\right)$