# Congruence Relation iff Compatible with Operation/Proof 2

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## Theorem

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

Let $\RR$ be an equivalence relation on $S$.

Then $\RR$ is a congruence relation for $\circ$ if and only if:

\(\displaystyle \forall x, y, z \in S: \ \ \) | \(\displaystyle x \mathrel \RR y\) | \(\implies\) | \(\displaystyle \paren {x \circ z} \mathrel \RR \paren {y \circ z}\) | ||||||||||

\(\displaystyle x \mathrel \RR y\) | \(\implies\) | \(\displaystyle \paren {z \circ x} \mathrel \RR \paren {z \circ y}\) |

That is, if and only if $\RR$ is compatible with $\circ$.

## Proof

We have that an equivalence relation is a (symmetric) preordering.

Thus the result Preordering of Products under Operation Compatible with Preordering can be applied directly.

$\blacksquare$