Closed Form for Heptagonal Numbers
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Theorem
The closed-form expression for the $n$th heptagonal number is:
- $H_n = \dfrac {n \paren {5 n - 3} } 2$
Proof
Heptagonal numbers are $k$-gonal numbers where $k = 7$.
From Closed Form for Polygonal Numbers we have that:
- $\map P {k, n} = \dfrac n 2 \paren {\paren {k - 2} n - k + 4}$
Hence:
\(\ds H_n\) | \(=\) | \(\ds \frac n 2 \paren {\paren {7 - 2} n - 7 + 4}\) | Closed Form for Polygonal Numbers | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {n \paren {5 n - 3} } 2\) |
Hence the result.
$\blacksquare$
Sources
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $81$