Sum of Odd Number of Odd Numbers is Odd
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Theorem
In the words of Euclid:
- If as many odd numbers as we please be added together, and their multitude is odd, the whole is odd.
(The Elements: Book $\text{IX}$: Proposition $23$)
Proof
Let $S = \set {r_1, r_2, \ldots, r_n}$ be a set of $n$ odd numbers, where $n = 2 m + 1$.
By definition of odd number, this can be expressed as:
- $S = \set {2 s_1 + 1, 2 s_2 + 1, \ldots, 2 s_n + 1}$
where:
- $\forall k \in \closedint 1 n: r_k = 2 s_k + 1$
Then:
\(\ds \sum_{k \mathop = 1}^n r_k\) | \(=\) | \(\ds \sum_{k \mathop = 1}^{2 m + 1} \paren {2 s_k + 1}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^{2 m} \paren {2 s_k + 1} + 2 s_n + 1\) | extracting the $n$th element from the summation | |||||||||||
\(\ds \) | \(=\) | \(\ds 2 q + 2 s_n + 1\) | where $q \in \N$: Sum of Even Number of Odd Numbers is Even | |||||||||||
\(\ds \) | \(=\) | \(\ds 2 \paren {q + s_n} + 1\) |
Thus, by definition, $\ds \sum_{k \mathop = 1}^n r_k$ is odd.
$\blacksquare$
Historical Note
This proof is Proposition $23$ of Book $\text{IX}$ of Euclid's The Elements.
Sources
- 1926: Sir Thomas L. Heath: Euclid: The Thirteen Books of The Elements: Volume 2 (2nd ed.) ... (previous) ... (next): Book $\text{IX}$. Propositions