Integers under Subtraction do not form Group

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 group.

Proof
It is to be demonstrated that $\struct {\Z, -}$ does not satisfy the group axioms.

First it is noted that Integer Subtraction is Closed.

Thus $\struct {\Z, -}$ fulfils.

However, 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 the group axioms.

Hence the result.