Cancellable Infinite Semigroup is not necessarily Group
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Theorem
Let $\struct {S, \circ}$ be a semigroup whose underlying set is infinite.
Let $\struct {S, \circ}$ be such that all elements of $S$ are cancellable.
Then it is not necessarily the case that $\struct {S, \circ}$ is a group.
Proof
Consider the semigroup $\struct {\N, +}$.
From Natural Numbers under Addition form Commutative Semigroup, $\struct {\N, +}$ forms a semigroup.
From Natural Numbers are Infinite, the underlying set of $\struct {\N, +}$ is infinite.
From Natural Number Addition is Cancellable, all elements of $\struct {\N, +}$ are cancellable.
But from Natural Numbers under Addition do not form Group, $\struct {\N, +}$ is not a group.
Sources
- 1978: Thomas A. Whitelaw: An Introduction to Abstract Algebra ... (previous) ... (next): Chapter $6$: An Introduction to Groups: Exercise $3$