2415

Number
$2415$ (two thousand, four hundred and fifteen) is:
 * $3 \times 5 \times 7 \times 23$


 * The $69$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $1891$, $1953$, $2016$, $2080$, $2145$, $2211$, $2278$, $2346$:
 * $2415 = \displaystyle \sum_{k \mathop = 1}^{69} k = \dfrac {69 \times \left({69 + 1}\right)} 2$


 * The $35$th hexagonal number after $1$, $6$, $15$, $28$, $45$, $66$, $91$, $\ldots$, $1431$, $1540$, $1653$, $1770$, $1891$, $2016$, $2145$, $2278$:
 * $2415 = \displaystyle \sum_{k \mathop = 1}^{35} \left({4 k - 3}\right) = 35 \left({2 \times 35 - 1}\right)$