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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

$\frac{5n^2 - 3n}{2}$.

The first few heptagonal numbers are:

1, 7, 18, 34, 55, 81, 112, 148, 189, 235, 283, 342, 403, 469, 540, 616, 645, 783, 874, 939, 1056, 11785, 1288, 1404, 1525, 1651, 1782, …

## Parity Edit

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.

## Sum of reciprocals Edit

A formula for the sum of the reciprocals of the heptagonal numbers is given by:

$\sum_{n=1}^\infty \frac{2}{n(5n-3)} = \frac{1}{15}{\pi}{\sqrt{25-10\sqrt{5}}}+\frac{2}{3}\ln(5)+\frac{{1}+\sqrt{5}}{3}\ln\left(\frac{1}{2}\sqrt{10-2\sqrt{5}}\right)+\frac{{1}-\sqrt{5}}{3}\ln\left(\frac{1}{2}\sqrt{10+2\sqrt{5}}\right)$

## Heptagonal roots Edit

In analogy to the square root of x, one can calculate the heptagonal root of x, meaning the number of terms in the sequence up to and including x.

The heptagonal root of x is given by the formula

$n = \frac{\sqrt{40x + 9} + 3}{10},$

which is obtained by using the quadratic formula to solve $x = \frac{5n^2 - 3n}{2}$ for its unique positive root n.

## Sources Edit

1. Beyond the Basel Problem: Sums of Reciprocals of Figurate Numbers
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