3321

Number
$3321$ (three thousand, three hundred and twenty-one) is:


 * $3^4 \times 41$


 * The $81$st triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $2701$, $2775$, $2850$, $2926$, $3003$, $3081$, $3160$, $3240$:
 * $3321 = \displaystyle \sum_{k \mathop = 1}^{81} k = \dfrac {81 \times \left({81 + 1}\right)} 2$


 * The $41$st hexagonal number after $1$, $6$, $15$, $28$, $45$, $66$, $91$, $\ldots$, $2145$, $2278$, $2415$, $2556$, $2701$, $2850$, $3003$, $3160$:
 * $3321 = \displaystyle \sum_{k \mathop = 1}^{41} \left({4 k - 3}\right) = 41 \left({2 \times 41 - 1}\right)$


 * The total of all the entries in a magic square of order $9$, after $1$, $(10)$, $45$, $136$, $325$, $666$, $1225$, $2080$:
 * $3321 = \displaystyle \sum_{k \mathop = 1}^{9^2} k = \dfrac {9^2 \paren {9^2 + 1} } 2$