697

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Number

$697$ (six hundred and ninety-seven) is:

$17 \times 41$

The $17$th heptagonal number after $1$, $7$, $18$, $34$, $55$, $81$, $112$, $148$, $189$, $235$, $286$, $342$, $403$, $469$, $540$, $616$:

$697 = \ds \sum_{k \mathop = 1}^{17} \paren {5 k - 4} = \dfrac {17 \paren {5 \times 17 - 3} } 2$

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