Diagonal Relation is Symmetric

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Theorem

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


Proof

\(\, \displaystyle \forall x, y \in S: \, \) \(\displaystyle \tuple {x, y}\) \(\in\) \(\displaystyle \Delta_S\) $\quad$ $\quad$
\(\displaystyle \leadsto \ \ \) \(\displaystyle x\) \(=\) \(\displaystyle y\) $\quad$ Definition of Diagonal Relation $\quad$
\(\displaystyle \leadsto \ \ \) \(\displaystyle y\) \(=\) \(\displaystyle x\) $\quad$ Equality is Symmetric $\quad$
\(\displaystyle \leadsto \ \ \) \(\displaystyle \tuple {y, x}\) \(\in\) \(\displaystyle \Delta_S\) $\quad$ Definition of Diagonal Relation $\quad$

So $\Delta_S$ is symmetric.

$\blacksquare$