Set of All Relations is a Monoid

Theorem
The set of all relations $$\mathbb R = \left\{{\mathcal R: \mathcal R \subseteq S \times S}\right\}$$ on a set $$S$$ forms a monoid in which the operation is composition of relations.

The only invertible elements of $$\mathbb R$$ are permutations.

If $$\mathcal R$$ is invertible, its inverse is $$\mathcal R^{-1}$$.

Closure
A relation followed by another relation is another relation, from the definition of composition of relations.

As the domain and codomain of two relations are the same, then:
 * $$\forall \mathcal R_1, \mathcal R_2 \subseteq S \times S: \mathcal R_1 \circ \mathcal R_2 \subseteq S \times S$$

Therefore composition of relations on a set is closed.

Associativity
Composition of relations is associative.

Identity
From Identity Mapping is Left Identity and Identity Mapping is Right Identity we have that:
 * $$\forall \mathcal R \subseteq S \times S: \mathcal R \circ I_S = \mathcal R = I_S \circ \mathcal R$$

Hence the identity mapping is the identity element of this set of relations.

It's closed and associative, so it's a semigroup. It has an identity element, so it's a monoid.

Inverses
Now if $$\mathcal R \in \mathbb R$$ were to be invertible, it would need to satisfy:
 * $$\mathcal R^{-1} \circ \mathcal R = I_S$$ and
 * $$\mathcal R \circ \mathcal R^{-1} = I_S$$.

From Inverse Relation is Left and Right Inverse iff Bijection this can only happen if $$\mathcal R$$ is a bijection on $S$, that is, a permutation, and its inverse is then $$\mathcal R^{-1}$$.