Least Fixed Point of Enumeration Operator is Recursively Enumerable
Theorem
Let $\psi : \powerset \N \to \powerset \N$ be an enumeration operator.
Then there exists a recursively enumerable set $A$ such that:
- $A$ is a fixed point of $\psi$
- Every fixed point of $\psi$ is a superset of $A$
Proof
By Least Fixed Point of Enumeration Operator, such an $A$ can be defined as:
- $\ds \bigcup_{i \mathop \in \N} A_i$
where:
- $A_0 = \O$
- $A_{n + 1} = \map \psi {A_n}$
By definition of enumeration operator, there exists a recursively enumerable set $\phi \subseteq \N$ such that:
- $\map \psi A = \set {x \in \N : \exists \text { finite } B \subseteq A : \map \pi {x, b} \in \phi}$
where:
- $b \in \N$ is the code number for $B$
- $\pi : \N^2 \to \N$ is the Cantor pairing function
Suppose that there is no $x_0 \in \N$ such that:
- $\map \pi {x_0, 0} \in \phi$
Then, for every $x \in \N$:
- $\map \pi {x, 0} \notin \phi$
By induction, we will show that every $A_i = \O$.
$A_0 = \O$ by definition.
If $A_i = \O$, then by Subset of Empty Set iff Empty, for every $B \subseteq A_i$:
- $B = \O$
Therefore:
- $b = 0$
But then:
- $\map \pi {x, b} \notin \phi$
for every $x \in \N$.
Thus, by definition of $A_{i + 1}$:
- $A_{i + 1} = \O$
By Principle of Mathematical Induction:
- $A_i = \O$
for every $i \in \N$.
Thus:
- $\ds A = \bigcup_{i \mathop \in \N} A_i = \empty$
As $\phi$ is recursively enumerable and $A = \phi$:
- $A$ is recursively enumerable
$\Box$
Now, suppose there is some $x_0 \in \N$ such that:
- $\map \pi {x_0, 0} \in \phi$
Then:
- $x_0 \in A_1 \subseteq A$
Thus, $A$ is non-empty.
By Corollary to Recursively Enumerable Set is Image of Primitive Recursive Function:
- There exists a primitive recursive function $f : \N \to \N$ such that:
- $\Img f = \phi$
Define:
- $\map g {t, b} = \begin{cases}
\map {\pi_1} {\map f t} & : \map {\RR_\subseteq} {\map {\pi_2} {\map f t}, b} \\ x_0 & : \text {otherwise} \end{cases}$ where:
- $\pi_1$ and $\pi_2$ are the projections of the Cantor pairing function
- $\map {\RR_\subseteq} {x, y}$ if and only if $x$ and $y$ respectively code finite sets $X$ and $Y$ such that $X \subseteq Y$
Work In Progress In particular: Need proof that $\RR_\subseteq$ is primitive recursive; in progress You can help $\mathsf{Pr} \infty \mathsf{fWiki}$ by completing it. To discuss this page in more detail, feel free to use the talk page. When this work has been completed, you may remove this instance of {{WIP}} from the code. |