Equivalence Class holds Equivalent Elements

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Theorem

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


Then:

$\tuple {x, y} \in \RR \iff \eqclass x \RR = \eqclass y \RR$


Proof

Necessary Condition

First we prove that $\tuple {x, y} \in \RR \implies \eqclass x \RR = \eqclass y \RR$.

Suppose:

$\tuple {x, y} \in \RR: x, y \in S$

Then:

\(\displaystyle z\) \(\in\) \(\displaystyle \eqclass x \RR\)
\(\displaystyle \leadsto \ \ \) \(\displaystyle \tuple {x, z}\) \(\in\) \(\displaystyle \RR\) Definition of Equivalence Class
\(\displaystyle \leadsto \ \ \) \(\displaystyle \tuple {z, x}\) \(\in\) \(\displaystyle \RR\) Definition of Equivalence Relation: $\RR$ is symmetric
\(\displaystyle \leadsto \ \ \) \(\displaystyle \tuple {z, y}\) \(\in\) \(\displaystyle \RR\) Definition of Equivalence Relation: $\RR$ is transitive
\(\displaystyle \leadsto \ \ \) \(\displaystyle \tuple {y, z}\) \(\in\) \(\displaystyle \RR\) Definition of Equivalence Relation: $\RR$ is symmetric
\(\displaystyle \leadsto \ \ \) \(\displaystyle z\) \(\in\) \(\displaystyle \eqclass y \RR\) Definition of Equivalence Class

So:

$\eqclass x \RR \subseteq \eqclass y \RR$


Now:

\(\displaystyle \tuple {x, y} \in \RR\) \(\implies\) \(\displaystyle \eqclass x \RR \subseteq \eqclass y \RR\) (see above)
\(\displaystyle \tuple {x, y} \in \RR\) \(\implies\) \(\displaystyle \tuple {y, x} \in \RR\) Definition of Equivalence Relation: $\RR$ is symmetric
\(\displaystyle \) \(\leadsto\) \(\displaystyle \eqclass y \RR \subseteq \eqclass x \RR\) from above
\(\displaystyle \) \(\leadsto\) \(\displaystyle \eqclass y \RR = \eqclass x \RR\) Definition of Set Equality


... so we have shown that:

$\tuple {x, y} \in \RR \implies \eqclass x \RR = \eqclass y \RR$.

$\Box$


Sufficient Condition

Next we prove that $\eqclass x \RR = \eqclass y \RR \implies \tuple {x, y} \in \RR$.

By definition of set equality:

$\eqclass x \RR = \eqclass y \RR$

means:

$\paren {x \in \eqclass x \RR \iff x \in \eqclass y \RR}$

So by definition of equivalence class:

$\tuple {y, x} \in \RR$

Hence by definition of equivalence relation: $\RR$ is symmetric

$\tuple {x, y} \in \RR$

So we have shown that

$\eqclass x \RR = \eqclass y \RR \implies \tuple {x, y} \in \RR$


Thus, we have:

\(\displaystyle \tuple {x, y} \in \RR\) \(\implies\) \(\displaystyle \eqclass x \RR = \eqclass y \RR\)
\(\displaystyle \eqclass x \RR = \eqclass y \RR\) \(\implies\) \(\displaystyle \tuple {x, y} \in \RR\)

$\Box$


So by equivalence:

$\tuple {x, y} \in \RR \iff \eqclass x \RR = \eqclass y\RR$

$\blacksquare$


Sources