# Number neither whose Half is Odd nor Power of Two is both Even-Times Even and Even-Times Odd

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## Theorem

Let $a \in \Z$ be an integer such that:

- $(1): \quad a$ is not a power of $2$
- $(2): \quad \dfrac a 2$ is an even integer.

Then $a$ is both even-times even and even-times odd.

In the words of Euclid:

*If a number neither be one of those which are continually doubled from a dyad, nor have its half odd, it is both even-times even and even-times odd.*

(*The Elements*: Book $\text{IX}$: Proposition $34$)

## Proof

As $\dfrac a 2$ is an even integer it follows that:

- $\dfrac a 2 = 2 r$

for some $r \in \Z$.

That is:

- $a = 2^2 r$

and so $a$ is even-times even by definition.

$\blacksquare$

Suppose $a$ is not even-times odd.

Then $a$ does not have an odd divisor.

Thus in its prime decomposition there are no odd primes.

Thus $a$ is in the form:

- $a = 2^k$

for some $k \in \Z_{>0}$.

That is, $a$ is a power of $2$.

This contradicts condition $(1)$.

From this contradiction it is deduced that $a$ is even-times odd.

$\blacksquare$

## Historical Note

This proof is Proposition $34$ 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