# Invertible Elements of Monoid form Subgroup of Cancellable Elements

## Theorem

Let $\struct {S, \circ}$ be an monoid whose identity is $e_S$.

Let $C$ be the set of all cancellable elements of $S$.

Let $T$ be the set of all invertible elements of $S$.

Then $\struct {T, \circ}$ is a subgroup of $\struct {C, \circ}$.

## Proof

From Cancellable Elements of Monoid form Submonoid, $\struct {C, \circ}$ is a submonoid of $\struct {S, \circ}$.

Let its identity be $e_C$ (which may or may not be the same as $e_S$).

Let $T$ be the set of all invertible elements of $S$.

From Invertible Element of Monoid is Cancellable, all the invertible elements of $S$ are also all cancellable, so $T \subseteq C$.

Let $x, y \in T$.

Clearly $x^{-1}, y^{-1} \in T$, as if $x, y$ are invertible, then so are their inverses.

Taking the group axioms in turn:

### Group Axiom $\text G 0$: Closure

By Inverse of Product, $x^{-1} \circ y^{-1} \in T$.

Thus $\struct {T, \circ}$ is closed.

$\Box$

### Group Axiom $\text G 1$: Associativity

This is inherited from $S$, by Subsemigroup Closure Test.

Thus $\struct {T, \circ}$ is associative.

$\Box$

### Group Axiom $\text G 2$: Existence of Identity Element

All the elements of $\struct {C, \circ}$ are by definition cancellable

So, by Identity of Cancellable Monoid is Identity of Submonoid:

- $e_C \in T$

Thus $\struct {T, \circ}$ has an identity element.

$\Box$

### Group Axiom $\text G 3$: Existence of Inverse Element

By Inverse of Inverse in Monoid:

- $\struct {x^{-1} \circ y^{-1} }^{-1} \in T$

Thus every element of $\struct {T, \circ}$ has an inverse.

$\Box$

All the group axioms are thus seen to be fulfilled, and so $\struct {T, \circ}$ is a group.

$\blacksquare$

## Sources

- 1965: J.A. Green:
*Sets and Groups*... (previous) ... (next): Chapter $4$. Groups: Exercise $13$ - 1965: Seth Warner:
*Modern Algebra*... (previous) ... (next): Chapter $\text {II}$: New Structures from Old: $\S 8$: Compositions Induced on Subsets: Theorem $8.5$