726

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Number

$726$ (seven hundred and twenty-six) is:

$2 \times 3 \times 11^2$

The $11$th pentagonal pyramidal number after $1$, $6$, $12$, $40$, $75$, $126$, $196$, $288$, $405$, $550$:

$726 = \ds \sum_{k \mathop = 1}^{11} \dfrac {k \paren {3 k - 1} } 2 = \dfrac {11^2 \paren {11 + 1} } 2$

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