2415

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Number

$2415$ (two thousand, four hundred and fifteen) is:

$3 \times 5 \times 7 \times 23$

The $35$th hexagonal number after $1$, $6$, $15$, $28$, $45$, $66$, $91$, $\ldots$, $1431$, $1540$, $1653$, $1770$, $1891$, $2016$, $2145$, $2278$:

$2415 = \ds \sum_{k \mathop = 1}^{35} \paren {4 k - 3} = 35 \paren {2 \times 35 - 1}$

The $69$th triangular number after $1$, $3$, $6$, $10$, $15$, $\ldots$, $1891$, $1953$, $2016$, $2080$, $2145$, $2211$, $2278$, $2346$:

$2415 = \ds \sum_{k \mathop = 1}^{69} k = \dfrac {69 \times \paren {69 + 1} } 2$

Also see

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