Integers under Subtraction do not form Semigroup
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Theorem
Let $\struct {\Z, -}$ denote the algebraic structure formed by the set of integers under the operation of subtraction.
Then $\struct {\Z, -}$ is not a semigroup.
Proof
It is to be demonstrated that $\struct {\Z, -}$ does not satisfy the semigroup axioms.
We then have Subtraction on Numbers is Not Associative.
So, for example:
- $3 - \paren {2 - 1} = 2 \ne \paren {3 - 2} - 1 = 0$
Thus it has been demonstrated that $\struct {\Z, -}$ does not satisfy Semigroup Axiom $\text S 1$: Associativity.
Hence the result.
$\blacksquare$
Sources
- 1970: B. Hartley and T.O. Hawkes: Rings, Modules and Linear Algebra ... (previous) ... (next): Chapter $1$: Rings - Definitions and Examples: Exercise $1$