2109
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Number
$2109$ (two thousand, one hundred and nine) is:
- $3 \times 19 \times 37$
- The $18$th square pyramidal number after $1$, $5$, $14$, $30$, $55$, $91$, $140$, $204$, $285$, $385$, $506$, $650$, $819$, $1015$, $1240$, $1496$, $1785$:
- $2109 = \ds \sum_{k \mathop = 1}^{18} k^2 = \dfrac {18 \paren {18 + 1} \paren {2 \times 18 + 1} } 6$