Let $A$, $B$, $C$ be Turing-recognizable languages over an alphabet $\Sigma$. Assume that
- $A\cup B\cup C = \Sigma^*$, and
- $A\cap B = A\cap C = B\cap C = \emptyset$.
Prove that $A$ is Turing-decidable.
So far I've tried the following but it really hasn't gotten me anywhere.
Suppose $A$ is recognizable but not decidable then, $\overline{A} = B \cup C$ so $B$ or $C$ is undecidable, since decidable languages are closed under union.
In the case that only one is decidable you can show at least that its in $A$ or $B$ (or $A$ or $C$ in the other case). Then, ... ?
In the case that both are recognizable, ... ?