Slow g-Tower is Slowly Well-Ordered under Subset Relation
Theorem
Let $M$ be a class.
Let $g: M \to M$ be a slowly progressing mapping on $M$.
Let $M$ be a slow $g$-tower.
Then $M$ is slowly well-ordered under the subset relation.
Proof
Let $M$ be a slow $g$-tower.
By $g$-Tower is Well-Ordered under Subset Relation: Empty Set:
- $\O$ is the smallest element of $M$.
This is condition $(\text S_1)$ of the definition of slowly well-ordered class under subset relation.
By $g$-Tower is Well-Ordered under Subset Relation: Union of Limit Elements:
- Each limit element $x$ of $M$ is the union of its lower section $\ds \bigcup x^\subset$
This is condition $(\text S_3)$ of the definition of slowly well-ordered class under subset relation.
It remains to establish condition $(\text S_2)$ of the definition of slowly well-ordered class under subset relation.
Let $x \in M$ be arbitrary.
Let $y$ be the immediate successor of $x$.
Then $x$ is not the greatest element of $M$.
Hence by $g$-Tower is Well-Ordered under Subset Relation: Successor of Non-Greatest Element:
- $y = \map g x$
Because $g$ is slowly progressing, $y$ contains at most $1$ more element than $x$.
But we also have that $y$ is the immediate successor of $x$.
Thus $x$ is a proper subset of $y$.
So $y$ contains at least $1$ more element than $x$.
Hence $y$ contains exactly $1$ element which is not in $x$.
The result follows.
$\blacksquare$
Sources
- 2010: Raymond M. Smullyan and Melvin Fitting: Set Theory and the Continuum Problem (revised ed.) ... (previous) ... (next): Chapter $4$: Superinduction, Well Ordering and Choice: Part $\text I$ -- Superinduction and Well Ordering: $\S 4$ Well ordering and choice: Theorem $4.3$