4005

Number
$4005$ (four thousand and five) is:


 * $3^2 \times 5 \times 89$


 * The $89$th triangular number:
 * $4005 = \displaystyle \sum_{k \mathop = 1}^{89} k = \dfrac {89 \left({89 + 1}\right)} 2$


 * The $45$th hexagonal number after $1$, $6$, $15$, $28$, $45$, $66$, $91$, $\ldots$, $2145$, $2278$, $2415$, $2556$, $2701$, $2850$, $3003$, $3160$, $3321$, $3486$, $3655$, $3828$:
 * $4005 = \displaystyle \sum_{k \mathop = 1}^{45} \left({4 k - 3}\right) = 45 \left({2 \times 45 - 1}\right)$