GCD for Negative Integers

Theorem
$$\gcd \left\{{a, b}\right\} = \gcd \left\{{\left|{a}\right|, b}\right\} = \gcd \left\{{a, \left|{b}\right|}\right\} = \gcd \left\{{\left|{a}\right|, \left|{b}\right|}\right\}$$

Alternatively, this can be put:

$$\gcd \left\{{a, b}\right\} = \gcd \left\{{-a, b}\right\} = \gcd \left\{{a, -b}\right\} = \gcd \left\{{-a, -b}\right\}$$ which follows directly from the above.

Proof
Note that $$\left|{a}\right| = \pm a$$.

Suppose $$u \backslash a$$. Then $$\exists q \in \mathbb{Z}: a = q u$$.

Then $$\left|{a}\right| = \pm q u = \left({\pm q}\right) u \Longrightarrow u \backslash \left|{a}\right|$$.

So every factor of $$a$$ is a factor of $$\left|{a}\right|$$. Similarly, note that $$a = \pm \left|{a}\right|$$, so every factor of $$\left|{a}\right|$$ is a factor of $$a$$.

So it follows that the common factors of $$a$$ and $$b$$ are the same as those of $$a$$ and $$\left|{b}\right|$$, etc., and in particular $$\gcd \left\{{a, b}\right\} = \gcd \left\{{a, \left|{b}\right|}\right\}$$ etc.