3311

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Number

$3311$ (three thousand, three hundred and eleven) is:

$7 \times 11 \times 43$

The $21$st square pyramidal number after $1$, $5$, $14$, $30$, $55$, $\ldots$, $1015$, $1240$, $1496$, $1785$, $2109$, $2470$, $2870$:

$3311 = \ds \sum_{k \mathop = 1}^{21} k^2 = \dfrac {21 \paren {21 + 1} \paren {2 \times 21 + 1} } 6$

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