Cancellable Finite Semigroup is Group

Theorem
Let $\left({S, \circ}\right)$ be a finite semigroup in which all elements are cancellable.

Then $\left({S, \circ}\right)$ is a group.

Proof
As $\left({S, \circ}\right)$ is a semigroup, it is already closed and associative.

It remains to be shown that it has an identity and that every element of $S$ has an inverse in $S$.

First we show that $\left({S, \circ}\right)$ has an identity.

Choose $a \in S$.

Let the mapping $\lambda_a: S \to S$ be the left regular representation of $\left({S, \circ}\right)$ with respect to $a$.

By hypothesis:
 * all elements of $S$ are cancellable


 * $S$ is finite.

By Regular Representation wrt Cancellable Element on Finite Semigroup is Bijection, $\lambda_a$ is a bijection.

Hence $a \circ e = a$ for some $e \in S$.

Let $x \in S$.

Then because of cancellability:

Thus $e$ is the identity.

The existence of inverses comes from the surjectivity of $\lambda_a$.

As $\lambda_a$ is surjective:
 * $\exists y \in S: \lambda_a \left({y}\right) = e$

That is:
 * $a \circ y = e$

So we see that $y$ acts as a right inverse for $a$.

This is the case for any $a \in S$: all of them have some right inverse.

So, from Right Inverse for All is Left Inverse, each of these elements is also a left inverse, and therefore an inverse.

Thus $S$ is closed, associative, has an identity and every element has an inverse.

So, by definition, $\left({S, \circ}\right)$ is a group.

Also see

 * Cancellable Infinite Semigroup is not necessarily Group