Heptagonal number
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A heptagonal number is a figurate number that represents a heptagon. The n-th heptagonal number is given by the formula
- n(5n - 3)/2.
The parity of heptagonal numbers follows the pattern odd-odd-even-even. Like square numbers, the digital root in base 10 of a heptagonal number can only be 1, 4, 7 or 9.
Five times a heptagonal number, plus 1 equals a triangular number.
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