861

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


 * $3 \times 7 \times 41$


 * The $41$st triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $325$, $351$, $378$, $406$, $435$, $465$, $496$, $528$, $561$, $595$, $630$, $666$, $703$, $741$, $780$, $820$:
 * $861 = \displaystyle \sum_{k \mathop = 1}^{41} k = \dfrac {41 \times \left({41 + 1}\right)} 2$


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