Closed Form for Pentagonal Pyramidal Numbers
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Theorem
The closed-form expression for the $n$th pentagonal pyramidal number is:
- $Q_n = \dfrac {n^2 \paren {n + 1} } 2$
Proof
\(\ds Q_n\) | \(=\) | \(\ds \sum_{k \mathop = 1}^n P_k\) | Definition of Pentagonal Pyramidal Number | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^n \dfrac {k \paren {3 k - 1} } 2\) | Closed Form for Pentagonal Numbers | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 2 \paren {3 \sum_{k \mathop = 1}^n k^2 - \sum_{k \mathop = 1}^n k}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 2 \paren {3 \frac {n \paren {n + 1} \paren {2 n + 1} } 6 - \sum_{k \mathop = 1}^n k}\) | Sum of Sequence of Squares | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 2 \paren {3 \frac {n \paren {n + 1} \paren {2 n + 1} } 6 - \dfrac {n \paren {n + 1} } 2}\) | Closed Form for Triangular Numbers | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 2 \paren {\frac {n \paren {n + 1} \paren {2 n + 1} - n \paren {n + 1} } 2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 2 \paren {\frac {n \paren {n + 1} \paren {2 n} } 2}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {n^2 \paren {n + 1} } 2\) |
$\blacksquare$
Sources
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $55$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $55$