Composite Relation with Inverse is Symmetric

Theorem
Let $\RR \subseteq S \times T$ be a relation.

Then the composition of $\RR$ with its inverse $\RR^{-1}$ is symmetric:


 * $\RR^{-1} \circ \RR$ is a symmetric relation on $S$
 * $\RR \circ \RR^{-1}$ is a symmetric relation on $T$.

Proof
Note that this result holds for any $\RR \subseteq S \times T$, and does not require that $\struct {S, \RR}$ necessarily be a relational structure.

Thus $\tuple {a, b} \in \RR^{-1} \circ \RR \implies \tuple {b, a} \in \RR^{-1} \circ \RR$ and thus $\RR^{-1} \circ \RR$ is symmetric.

As $\RR = \paren {\RR^{-1} }^{-1}$ from Inverse of Inverse Relation, it follows that $\RR \circ \RR^{-1} = \paren {\RR^{-1} }^{-1} \circ \RR^{-1}$ is likewise a symmetric relation.

The domain of $\RR^{-1} \circ \RR$ is $S$ from Domain of Composite Relation, as is its codomain from Codomain of Composite Relation and the definition of Inverse Relation.

Similarly, the codomain of $\RR \circ \RR^{-1}$ is $T$, as is its domain.

This completes the proof.