Category of Relations is Category
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Theorem
Let $\mathbf{Rel}$ be the category of relations.
Then $\mathbf{Rel}$ is a metacategory.
Proof
Let us verify the axioms $(C1)$ up to $(C3)$ for a metacategory.
We have already defined the composition of morphisms.
For any set $X$, we have the diagonal relation $\operatorname{id}_X$.
By Diagonal Relation is Left Identity and Diagonal Relation is Right Identity it follows that this is the identity morphism for $X$.
Finally by Composition of Relations is Associative, the associative property is satisfied.
Hence $\mathbf{Rel}$ is a metacategory.
$\blacksquare$
Sources
- 2010: Steve Awodey: Category Theory (2nd ed.) ... (previous) ... (next): $\S 1.9$: Exercise $1 \ \text{(a)}$