Sum of Sequence of Fifth Powers
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Theorem
- $\ds \sum_{i \mathop = 1}^n i^5 = \dfrac { {T_n}^2 \paren {4 T_n - 1} } 3$
where $T_n$ denotes the $n$th triangular number.
Proof
The proof proceeds by induction.
For all $n \in \Z_{> 0}$, let $\map P n$ be the proposition:
- $\ds \sum_{i \mathop = 1}^n i^5 = \dfrac { {T_n}^2 \paren {4 T_n - 1} } 3$
Basis for the Induction
$\map P 1$ is the case:
\(\ds \dfrac { {T_1}^2 \paren {4 T_1 - 1} } 3\) | \(=\) | \(\ds \dfrac {\paren {\frac {1 \paren {1 + 1} } 2}^2 \paren {4 \frac {1 \paren {1 + 1} } 2 - 1} } 3\) | Closed Form for Triangular Numbers | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {1^2 \times \paren {4 \times 1 - 1} } 3\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 1\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{i \mathop = 1}^1 i^5\) |
Thus $\map P 1$ is seen to hold.
This is the basis for the induction.
Induction Hypothesis
Now it needs to be shown that, if $\map P k$ is true, where $k \ge 2$, then it logically follows that $\map P {k + 1}$ is true.
So this is the induction hypothesis:
- $\ds \sum_{i \mathop = 1}^k i^5 = \dfrac { {T_k}^2 \paren {4 T_k - 1} } 3$
from which it is to be shown that:
- $\ds \sum_{i \mathop = 1}^{k + 1} i^5 = \dfrac { {T_{k + 1} }^2 \paren {4 T_{k + 1} - 1} } 3$
Induction Step
This is the induction step:
\(\ds \sum_{i \mathop = 1}^{k + 1} i^5\) | \(=\) | \(\ds \sum_{i \mathop = 1}^k i^5 + \paren {k + 1}^5\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac { {T_k}^2 \paren {4 T_k - 1} } 3 + \paren {k + 1}^5\) | Induction Hypothesis | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {\paren {\frac {k \paren {k + 1} } 2}^2 \paren {4 \frac {k \paren {k + 1} } 2 - 1} } 3 + \paren {k + 1}^5\) | Closed Form for Triangular Numbers | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {\paren {k \paren {k + 1} }^2 \paren {2 k \paren {k + 1} - 1} } {12} + \paren {k + 1}^5\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {k + 1}^2 \dfrac {\paren {2 k^3 \paren {k + 1} - k^2} + 12 \paren {k + 1}^3} {12}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {k + 1}^2 \dfrac {2 k^4 + 2 k^3 - k^2 + 12 k^3 + 36 k^2 + 36 k + 12} {12}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {k + 1}^2 \dfrac {2 k^4 + 14 k^3 + 35 k^2 + 36 k + 12} {12}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {k + 1}^2 \paren {k + 2}^2 \dfrac {2 k^2 + 6 k + 3} {12}\) | extracting $\paren {k + 2}^2$ as a factor | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {\frac {\paren {k + 1} \paren {k + 2} } 2}^2 \dfrac {4 \frac {\paren {k + 1} \paren {k + 2} } 2 - 1} 3\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac { {T_{k + 1} }^2 \paren {4 T_{k + 1} - 1} } 3\) | Closed Form for Triangular Numbers |
So $\map P k \implies \map P {k + 1}$ and the result follows by the Principle of Mathematical Induction.
Therefore:
- $\forall n \in \Z_{> 0}: \ds \sum_{i \mathop = 1}^n i^5 = \dfrac { {T_n}^2 \paren {4 T_n - 1} } 3$
$\blacksquare$
Also see
Sources
- 1986: David Wells: Curious and Interesting Numbers ... (previous) ... (next): $15$
- 1997: David Wells: Curious and Interesting Numbers (2nd ed.) ... (previous) ... (next): $15$