Subtraction on Numbers is Anticommutative
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Theorem
The operation of subtraction on the numbers is anticommutative.
That is:
- $a - b = b - a \iff a = b$
Proof
Natural Numbers
$a - b$ is defined on $\N$ only if $a \ge b$.
If $a > b$, then although $a - b$ is defined, $b - a$ is not.
So for $a - b = b - a$ it is necessary for both to be defined.
This happens only when $a = b$.
Hence the result.
Integers, Rationals, Reals, Complex Numbers
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:
- $a = b$
$\blacksquare$
Also see
Sources
- 1965: J.A. Green: Sets and Groups ... (previous) ... (next): $\S 4.2$. Commutative and associative operations: Example $61$
- 1967: Michael Spivak: Calculus ... (previous) ... (next): Part $\text I$: Prologue: Chapter $1$: Basic Properties of Numbers
- 1989: Ephraim J. Borowski and Jonathan M. Borwein: Dictionary of Mathematics ... (previous) ... (next): commutative or permutable: 1. (of an operator)