If Q1 and Q2 are countably enumerable, then is Q1\Q2 countably enumerable?
I am reading a text where they claim that this is not the case and ask the reader to come up with a counter example.
Can someone give a counter example to the claim that if both Q1 and Q2 are c.e. then so is Q1\Q2?
Since H is semi-decidable and hence also c.e., I was thinking of coming up with examples using it, but no success till now.