Integers under Multiplication do not form Group

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The set of integers under multiplication $\struct {\Z, \times}$ does not form a group.


In order to be classified as a group, the algebraic structure $\struct {\Z, \times}$ needs to fulfil the group axioms.

From Integers under Multiplication form Monoid, $\struct {\Z, \times}$ forms a monoid.

Therefore Group Axiom $\text G 0$: Closure, Group Axiom $\text G 1$: Associativity and Group Axiom $\text G 2$: Existence of Identity Element are satisfied.

However, from Invertible Integers under Multiplication, the only integers with inverses under multiplication are $1$ and $-1$.

As not all integers have inverses, it follows that $\struct {\Z, \times}$ is not a group.