Equivalent Characterizations of Finer Equivalence Relation
Theorem
Let $X$ be a set.
Let $\equiv$ and $\sim$ be equivalence relations on $X$.
The following statements are equivalent:
- $\equiv$ is finer than $\sim$:
- $\forall x, y \in X : x \equiv y \implies x \sim y$
- The graph of $\equiv$ is contained in the graph of $\sim$.
- Every $\equiv$-equivalence class is contained in a $\sim$-equivalence class.
- Every $\sim$-equivalence class is saturated under $\equiv$.
Proof
1 implies 2
\(\ds \tuple {x, y}\) | \(\in\) | \(\ds \map \TT \equiv\) | ||||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds x\) | \(\equiv\) | \(\ds y\) | Definition of Graph of Relation | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds x\) | \(\sim\) | \(\ds y\) | from $1$ | ||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \tuple {x, y}\) | \(\in\) | \(\ds \map \TT \sim\) | Definition of Graph of Relation |
By definition of subset, $\map \TT \equiv \subseteq \map \TT \sim$.
$\Box$
2 implies 1
\(\ds x\) | \(\equiv\) | \(\ds y\) | Definition of Equivalence Class | |||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds \tuple {x, y}\) | \(\in\) | \(\ds \map \TT \equiv\) | Definition of Graph of Relation | ||||||||||
\(\ds \) | \(\subseteq\) | \(\ds \map \TT \sim\) | from $2$ | |||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds x\) | \(\sim\) | \(\ds y\) | Definition of Graph of Relation |
$\Box$
1 implies 3
Let $x \in X$.
Then:
\(\ds y\) | \(\in\) | \(\ds \eqclass x \equiv\) | ||||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds y\) | \(\equiv\) | \(\ds x\) | Definition of Equivalence Class | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds y\) | \(\sim\) | \(\ds x\) | from $1$ | ||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds y\) | \(\in\) | \(\ds \eqclass x \sim\) | Definition of Equivalence Class |
By definition of subset, $\eqclass x \equiv \subseteq \eqclass x \sim$.
$\Box$
3 implies 1
For this proof and the next, we use this result implied by $3$:
Let $y \in X$.
$3$ implies:
- $\exists z \in X: \eqclass y \equiv \subseteq \eqclass z \sim$
By Element in its own Equivalence Class:
- $y \in \eqclass y \equiv \subseteq \eqclass z \sim$
- $y \in \eqclass y \sim$
By contrapositive of Equivalence Classes are Disjoint:
- $\eqclass y \sim = \eqclass z \sim$
Hence $\eqclass y \equiv \subseteq \eqclass y \sim$.
Thus:
\(\ds x\) | \(\equiv\) | \(\ds y\) | ||||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds x\) | \(\in\) | \(\ds \eqclass y \equiv\) | Definition of Equivalence Class | ||||||||||
\(\ds \) | \(\subseteq\) | \(\ds \eqclass y \sim\) | ||||||||||||
\(\ds \leadstoandfrom \ \ \) | \(\ds x\) | \(\sim\) | \(\ds y\) | Definition of Equivalence Class |
$\Box$
3 implies 4
Let $x \in X$.
\(\ds \bigcup_{y \mathop \in \eqclass x \sim} \eqclass y \equiv\) | \(\subseteq\) | \(\ds \bigcup_{y \mathop \in \eqclass x \sim} \eqclass y \sim\) | Set Union Preserves Subsets | |||||||||||
\(\ds \) | \(=\) | \(\ds \bigcup_{\substack {y \mathop \in X \\ y \mathop \sim x} } \eqclass y \sim\) | Definition of Equivalence Class | |||||||||||
\(\ds \) | \(=\) | \(\ds \bigcup_{\substack {y \mathop \in X \\ y \mathop \sim x} } \eqclass x \sim\) | Equivalence Class holds Equivalent Elements | |||||||||||
\(\ds \) | \(=\) | \(\ds \eqclass x \sim\) |
so $\eqclass x \sim$ is a union of equivalence classes under $\equiv$.
Hence $\eqclass x \sim$ is saturated under $\equiv$.
$\Box$
4 implies 3
Let $x \in X$.
Then $\eqclass x \sim$ is saturated under $\equiv$:
- $\ds \exists S \subseteq X: \bigcup_{y \mathop \in S} \eqclass y \equiv = \eqclass x \sim$
By Element in its own Equivalence Class:
- $x \in \eqclass x \sim$
- $x \in \eqclass x \equiv$
Hence by definition of Union of Family:
- $\exists z \in S: x \in \eqclass z \equiv$
By contrapositive of Equivalence Classes are Disjoint:
\(\ds \eqclass x \equiv\) | \(=\) | \(\ds \eqclass z \equiv\) | ||||||||||||
\(\ds \) | \(\subseteq\) | \(\ds \bigcup_{y \mathop \in S} \eqclass y \equiv\) | Set is Subset of Union of Family | |||||||||||
\(\ds \) | \(=\) | \(\ds \eqclass x \sim\) |
$\blacksquare$
Sources
- 1986: Nicolas Bourbaki: Theory of Sets ... (previous) Chapter $\text I$: Description of Formal Mathematics: $\S 6$ Equivalence relations $7$. Quotients of equivalence relations