Characterization of Strictly Increasing Mapping on Woset

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Theorem

Let $J$ and $E$ be well-ordered sets.

Let $h: J \to E$ be a mapping.

Let $S_\alpha$ denote an initial segment determined by $\alpha$.


The following statements are equivalent:

$(1): \quad h$ is strictly increasing and its image is either all of $E$ or an initial segment of $E$
$(2): \quad \forall \alpha \in J: \map h \alpha = \map \min {E \setminus h \sqbrk {S_\alpha} }$, and $h \sqbrk {S_\alpha} = S_{\map h \alpha}$

where:

$h \sqbrk {S_\alpha}$ denotes the image of $S_\alpha$ under $h$
$\min$ denotes the smallest element of the set.


Proof

$(1)$ implies $(2)$

Suppose $h$ satisfies:

$h$ is strictly increasing and its image is either all of $E$ or an initial segment of $E$

Then for any $x,y \in J$:

\(\ds x\) \(\prec\) \(\ds y\)
\(\ds \leadsto \ \ \) \(\ds \map h x\) \(\prec\) \(\ds \map h y\) Definition of Strictly Increasing Mapping
\(\ds h \sqbrk {S_y}\) \(=\) \(\ds \set {\map h x \in E: \exists x \in J: \map h x \prec \map h y}\)
\(\ds \) \(=\) \(\ds S_{\map h y}\)
\(\ds \map \min {E \setminus h \sqbrk {S_y} }\) \(=\) \(\ds \map \min {E \setminus S_{\map h y} }\)
\(\ds \) \(=\) \(\ds \map h y\) Definition of Smallest Element and Definition of Initial Segment

$\Box$


$(2)$ implies $(1)$

Suppose $h$ satisfies:

$\map h \alpha = \map \min {E \setminus h \sqbrk {S_\alpha} }$

By the Principle of Recursive Definition for Well-Ordered Sets, $h$ is thus uniquely determined.


Then:

\(\ds \map h y\) \(=\) \(\ds \map \min {E \setminus h \sqbrk {S_y} }\)
\(\ds h \sqbrk {S_y}\) \(=\) \(\ds \set {\map h x \in E: \exists x \in J: \map h x = \map \min {E \setminus h \sqbrk {S_y} } }\)
\(\ds \) \(=\) \(\ds \set {\map h x \in E: \exists x \in J: \map h x \prec \map h y}\)
\(\ds \) \(=\) \(\ds S_{\map h y}\) Definition of initial segment

Thus for every $x \in S_y$, we have that $\map h x \in S_{\map h y}$.

Therefore $h$ is an strictly increasing mapping.

Furthermore, the image set of $h$ is the union of initial segments in $E$.

By Union of Initial Segments is Initial Segment or All of Woset, $h \sqbrk J$ is an initial segment of $E$ or all of $E$.

$\blacksquare$


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