3781

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Number

$3781$ (three thousand, seven hundred and eighty-one) is:

$19 \times 199$

The $36$th centered hexagonal number after $1$, $7$, $19$, $37$, $61$, $91$, $127$, $\ldots$, $2791$, $2977$, $3169$, $3367$, $3571$:

$3781 = \ds 1 + \sum_{k \mathop = 1}^{36 - 1} 6 k = 36^3 - 35^3$

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