how do I find a undecidable subset of a set that's decidable? Given that Let S = {a | |a| is odd}. I know that since S is decidable, but does there exist a subset within S that is undecidable?
Hint: For every language $L$, the language $M = \{ 1^{2x+1} : x \in L \}$ is a subset of $S$.