Woset is Isomorphic to Set of its Initial Segments
Theorem
Let $\struct {S, \preceq}$ be a well-ordered set.
Let:
- $A = \set {a^\prec: a \in S}$
where $a^\prec$ is the strict lower closure of $S$ determined by $a$.
Then:
- $\struct {S, \preceq} \cong \struct {A, \subseteq}$
where $\cong$ denotes order isomorphism.
Proof
Define $f: S \to A$ as:
- $\forall a \in S: \map f a = a^\prec$
where $a^\prec$ is the initial segment determined by $a$.
$f$ is Surjective
$f$ is trivially surjective by the definition of $A$.
$\Box$
$f$ is Strictly Increasing
Let $x, y \in S$ with $x \prec y$.
Let $z \in \map f x$.
Then by the definition of initial segment:
- $z \prec x$
By Reflexive Reduction of Ordering is Strict Ordering, $\prec$ is also transitive.
Thus:
- $z \prec y$
Thus by the definition of initial segment:
- $z \in y^\prec = \map f y$
As this holds for all such $z$:
- $\map f x \subseteq \map f y$
As $x \prec y$:
- $x \in y^\prec = \map f y$
But since $\prec$ is antireflexive:
- $x \nprec x$
so:
- $x \notin \map f x$
Thus:
- $\map f x \subsetneqq \map f y$
As this holds for all such $x$ and $y$, $f$ is strictly increasing.
$\Box$
Since a well-ordering is a total ordering, $f$ is an order embedding by Mapping from Totally Ordered Set is Order Embedding iff Strictly Increasing.
Thus $f$ is a surjective order embedding and therefore an order isomorphism.
$\blacksquare$
Sources
- 1993: Keith Devlin: The Joy of Sets: Fundamentals of Contemporary Set Theory (2nd ed.) ... (previous) ... (next): $\S 1$: Naive Set Theory: $\S 1.7$: Well-Orderings and Ordinals: Theorem $1.7.5$