Union of Recursively Enumerable Sets
Theorem
Let $S, T \subseteq \N$ be recursively enumerable sets.
Then $S \cup T$ is recursively enumerable.
Proof
Suppose $S = \empty$.
Then by Union with Empty Set:
- $S \cup T = T$
which is recursively enumerable by assumption.
$\Box$
Suppose $T = \empty$.
For the same reason:
- $S \cup T = S$
which is also recursively enumerable by assumption.
$\Box$
Now, suppose neither $S$ nor $T$ is empty.
By Recursively Enumerable Set is Image of Primitive Recursive Function/Corollary, there exist primitive recursive $f, g : \N \to \N$ such that:
- $S = \Img f$
- $T = \Img g$
Define:
- $\map h {x, c} = \begin{cases}
\map f x & : c = 0 \\ \map g x & : \text{otherwise} \end{cases}$
By:
$h$ is a primitive recursive function.
By Primitive Recursive Function is Total Recursive Function, it is thus a recursive function.
It remains to be shown that $S \cup T = \Img h$.
Let $x \in S \cup T$.
Then $x \in \Img f$ or $x \in \Img g$.
If $x \in \Img f$, then there is some $z \in \N$ such that $\map f z = x$.
Therefore, $\map h {z, 0} = x$.
Likewise, if $x \in \Img g$, there is some $z \in \N$ such that $\map g z = x$.
Therefore, $\map h {z, 1} = x$.
Thus, $S \cup T \subseteq \Img h$.
Now, $x \in \Img h$.
Then, there are $z, c \in \N$ such that:
- $\map h {z, c} = x$
Depending on $c$, either:
- $x = \map f z$
or:
- $x = \map g z$
In either case, $x \in \Img f \cup \Img g$.
Therefore, $\Img h \subseteq S \cup T$.
Hence, by definition of set equality:
- $S \cup T = \Img h$
Therefore, by definition, $S \cup T$ is recursively enumerable.
$\blacksquare$