# Diagonal Relation is Universally Compatible

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## Theorem

The diagonal relation $\Delta_S$ on a set $S$ is universally compatible with every operation on $S$.

## Proof

Let $\struct {S, \circ}$ be any algebraic structure.

\(\ds \) | \(\) | \(\ds x_1 \Delta_S x_2 \land y_1 \Delta_S y_2\) | ||||||||||||

\(\ds \) | \(\leadsto\) | \(\ds x_1 = x_2 \land y_1 = y_2\) | Definition of Diagonal Relation | |||||||||||

\(\ds \) | \(\leadsto\) | \(\ds x_1 \circ y_1 = x_2 \circ y_2\) | (consequence of equality) | |||||||||||

\(\ds \) | \(\leadsto\) | \(\ds \paren {x_1 \circ y_1} \mathrel {\Delta_S} \paren {x_2 \circ y_2}\) | Definition of Diagonal Relation |

$\blacksquare$