990

Number
$990$ (nine hundred and ninety) is:


 * $2 \times 3^2 \times 5 \times 11$


 * The $44$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $496$, $528$, $561$, $595$, $630$, $666$, $703$, $741$, $780$, $820$, $861$, $903$, $946$:
 * $990 = \displaystyle \sum_{k \mathop = 1}^{44} k = \dfrac {44 \times \left({44 + 1}\right)} 2$


 * The second of the $4$th pair of triangular numbers whose sum and difference are also both triangular:
 * $780 = T_{39}$, $990 = T_{44}$, $780 + 990 = T_{59}$, $990 - 780 = T_{20}$