# Subtraction on Numbers is Anticommutative/Integral Domains

## Theorem

The operation of subtraction on the numbers is anticommutative.

That is:

$a - b = b - a \iff a = b$

## Proof

Let $a, b$ be elements of one of the standard number sets: $\Z, \Q, \R, \C$.

Each of those systems is an integral domain, and so is closed under the operation of subtraction.

### Necessary Condition

Let $a = b$.

Then $a - b = 0 = b - a$.

$\Box$

### Sufficient Condition

Let $a - b = b - a$.

Then:

 $\ds a - b$ $=$ $\ds a + \paren {-b}$ Definition of Subtraction $\ds$ $=$ $\ds \paren {-b} + a$ Commutative Law of Addition $\ds$ $=$ $\ds \paren {-b} + \paren {-\paren {-a} }$ $\ds$ $=$ $\ds -\paren {b - a}$ Definition of Subtraction

We have that:

$a - b = b - a$

So from the above:

$b - a = - \paren {b - a}$

That is:

$b - a = 0$

and so:

$a = b$

$\blacksquare$