Decidability in CFG

Question

As we know that equivalence in CSL is undecidable,so if i want to look for Non-equivalence,then will it be complement of this and answer will be NOT RE or will it be RE BUT NOT REC.

Similarly for disjointness test,if i say i want to see if some string is common in two CFG,will it be RE BUT NOT REC or NOT RE

Comments

@Saurabh

They have said equivalence of CFG is undecidable. So 2 cases:

1. Equivalence of CFG is RE but not recursive

    Now, complement of this cant be RE . So, complement is not RE

2. Equivalence of CFG isnt RE

    So, complement can be RE(but not recursive) or not RE

 

Right?

---

@sushant case i is true only

---

ok, thanks