721

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Number

$721$ (seven hundred and twenty-one) is:

$7 \times 103$

The $16$th centered hexagonal number after $1$, $7$, $19$, $37$, $61$, $91$, $127$, $169$, $217$, $271$, $331$, $397$, $469$, $547$, $631$:

$721 = \ds 1 + \sum_{k \mathop = 1}^{16 - 1} 6 k = 16^3 - 15^3$

Also see

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