Division Theorem/Positive Divisor/Existence/Proof 1

Proof
From Division Theorem: Positive Divisor: Positive Dividend: Existence:
 * $\forall a, b \in \Z, a \ge 0, b > 0: \exists q, r \in \Z: a = q b + r, 0 \le r < b$

That is, the result holds for positive $a$.

It remains to be shown that the statement holds for $a < 0$.

From Division Theorem: Positive Divisor: Positive Dividend:
 * $\exists \tilde q, \tilde r \in \Z: \size a = \tilde q b + \tilde r, 0 \le \tilde r < b$

where $\size a$ denotes the absolute value of $a$: by definition $\size a \ge 0$.

As $a < 0$ it follows by definition of absolute value that $\size a = -a$.

This gives:

Let $\tilde r = 0$.

Then:
 * $q = -\tilde q, r = \tilde r = 0$

which gives:
 * $a = q b + r, 0 \le r < b$

as required.

Otherwise:
 * $0 < \tilde r < b \implies 0 < b - \tilde r < b$

which suggests a rearrangement of the expression for $a$ above:

Taking:
 * $q = \paren {-1 - \tilde q}$

and:
 * $r = \paren {b - \tilde r}$

the required result follows.

Uniqueness of $q$ and $r$ follow from the Uniqueness of $\tilde q$ and $\tilde r$.