2109

From ProofWiki
Jump to navigation Jump to search

Previous  ... Next

Number

$2109$ (two thousand, one hundred and nine) is:

$3 \times 19 \times 37$


The $18$th square pyramidal number after $1$, $5$, $14$, $30$, $55$, $91$, $140$, $204$, $285$, $385$, $506$, $650$, $819$, $1015$, $1240$, $1496$, $1785$:
$2109 = \ds \sum_{k \mathop = 1}^{18} k^2 = \dfrac {18 \paren {18 + 1} \paren {2 \times 18 + 1} } 6$


Also see