Woset is Isomorphic to Set of its Initial Segments

Theorem
Let $\left({S, \preceq}\right)$ be a well-ordered set.

Let $A = \left\{{S_a: a \in S}\right\}$ where $S_a$ is the initial segment of $S$ determined by $a$.

Then:
 * $\left({S, \preceq}\right) \cong \left({A, \subseteq}\right)$

where $\cong$ denotes order isomorphism.

Proof
Define $f: S \to A$ as:
 * $\forall a \in S: f \left({a}\right) = S_a$

where $S_a$ is the initial segment determined by $a$.

$f$ is surjective
$f$ is trivially surjective by the definition of $A$.

$f$ is strictly increasing
Let $x,y \in S$ with $x \prec y$.

Let $z \in f(x)$.

Then $z \prec x$ by the definition of initial segment.

By Reflexive Reduction of Ordering is Strict Ordering, $\prec$ is also transitive.

Thus $z \prec y$.

Thus by the definition of initial segment, $z \in S_y = f(y)$.

As this holds for all such $z$, $f(x) \subseteq f(y)$.

Since $x \prec y$, $x \in S_y = f(y)$.

But since $\prec$ is antireflexive, $x \nprec x$, so $x \notin f(x)$.

Thus $f(x) \subsetneqq f(y)$.

As this holds for all such $x$ and $y$, $f$ is strictly increasing.

Since a well-ordering is a total ordering, $f$ is an order embedding by Mapping from Toset is Order Embedding iff Strictly Increasing.

Thus $f$ is a surjective order embedding and therefore an order isomorphism.