For A,B that are not decidable, does AB U BA not necessarily decidable?
I think that the answer is NO. Not necessarily. I thought about the following example, but it does not refute exactly:
If we take A and A complement, and without losing generality the empty word is in A, than every word in A' is also in AA' U A'A, but I can't use it to show that every word in A is also in AA' U A'A.
If someone can help me how can I show that also A is in AA' U A'A the problem will be solved.
thank you very much.