Equivalence of Definitions of Saturated Set Under Equivalence Relation

Theorem

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

Let $T\subset S$ be a subset.

The following definitions of the concept of saturated set in the context of Equivalence Relation are equivalent:

Definition 1

$T$ is saturated if and only if it equals its saturation:

$T = \overline T$

Definition 2

$T$ is saturated if and only if it is a union of equivalence classes:

$\displaystyle \exists U \subset S : T = \bigcup_{u \mathop \in U} \left[\!\left[{u}\right]\!\right]$

Definition 3

$T$ is saturated if and only if it is the preimage of some set under the quotient mapping:

$\exists V \subset S / \sim \; : T = q^{-1} \left[{V}\right]$

Proof

1 implies 2

Let $T = \overline T$.

By definition of saturation:

$T = \displaystyle \bigcup_{t \mathop \in T} \left[\!\left[{t}\right]\!\right]$

so we can take $U = T$.

$\blacksquare$

1 implies 3

Let $T = \overline T$.

By definition of saturation:

$T = q^{-1} \left[{q \left[{T}\right]}\right]$

so we can take $V = q \left[{T}\right]$.

$\blacksquare$

2 implies 1

Let $T = \displaystyle\bigcup_{u \mathop \in U} \left[\!\left[{u}\right]\!\right]$ with $U \subset S$.

Let $s \in S$ and $t \in T$ such that $s \sim t$.

By definition of union:

$\exists u \in U : t \in \left[\!\left[{u}\right]\!\right]$

By definition of equivalence class:

$t \sim u$

Because $\sim$ is transitive:

$s \sim u$

By definition of equivalence class:

$s \in \left[\!\left[{u}\right]\!\right]$

Thus:

$s \in T$

Because $s$ was arbitrary:

$\overline T \subset T$
$T \subset \overline T$

Thus:

$T = \overline T$

$\blacksquare$

3 implies 1

Let $V$ be a subset of the quotient mapping of $S$ by $\sim$:

$V \subset S / \sim$

Let $T$ be the preimage of $V$ under $q$:

$T = q^{-1} \left[{V}\right]$
$q \left[{q^{-1} \left[{V}\right]}\right] = V$

Thus:

 $\displaystyle q^{-1} \left[{q \left[{T}\right]}\right]$ $=$ $\displaystyle q^{-1} \left[{q \left[{q^{-1} \left[{V}\right]}\right]}\right]$ $\displaystyle$ $=$ $\displaystyle q^{-1} \left[{V}\right]$ $\displaystyle$ $=$ $\displaystyle T$

Thus $T$ equals its saturation.

$\blacksquare$