403

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Number

$403$ (four hundred and three) is:

$13 \times 31$

The $13$th heptagonal number after $1$, $7$, $18$, $34$, $55$, $81$, $112$, $148$, $189$, $235$, $286$, $342$:

$403 = 1 + 7 + 11 + 16 + 21 + 26 + 31 + 36 + 41 + 46 + 51 + 56 = \dfrac {13 \paren {5 \times 13 - 3} } 2$

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