G.366 number half odd — Heath

IX.33

If a number have its half odd, it is even-times odd only.

For let the number A have its half odd;

I say that A is even-times odd only.

Now that it is even-times odd is manifest; for the half of it, being odd, measures it an even number of times. [G.365]

I say next that it is also even-times odd only.

For, if A is even-times even also,

it will be measured by an even number according to an even number; [G.363]

so that the half of it will also be measured by an even number though it is odd: which is absurd.

Therefore A is even-times odd only.

Q. E. D.

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