Diagonal Relation is Transitive

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Theorem

The diagonal relation $\Delta_S$ on a set $S$ is a transitive relation in $S$.


Proof

\(\, \displaystyle \forall x, y, z \in S: \, \) \(\displaystyle \tuple {x, y}\) \(\in\) \(\displaystyle \Delta_S \land \tuple {y, z} \in \Delta_S\)
\(\displaystyle \leadsto \ \ \) \(\displaystyle x\) \(=\) \(\displaystyle y \land y = z\) Definition of Diagonal Relation
\(\displaystyle \leadsto \ \ \) \(\displaystyle x\) \(=\) \(\displaystyle z\) Equality is Transitive
\(\displaystyle \leadsto \ \ \) \(\displaystyle \tuple {x, z}\) \(\in\) \(\displaystyle \Delta_S\) Definition of Diagonal Relation

So $\Delta_S$ is transitive.

$\blacksquare$