819

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Number

$819$ (eight hundred and nineteen) is:

$3^2 \times 7 \times 13$

The $13$th square pyramidal number after $1$, $5$, $14$, $30$, $55$, $91$, $140$, $204$, $285$, $385$, $506$, $650$:

$819 = \ds \sum_{k \mathop = 1}^{13} k^2 = \dfrac {13 \paren {13 + 1} \paren {2 \times 13 + 1} } 6$

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