how to get the set of the two ends of the relation in set theory

Question

Here, I have a relation r ∈ A ↔ B, and if I only know about r, how can I get the set A and B? Is it possible？

Actually, I doubt whether it can be done according to what I know, and if it's impossible ,I need to find some new methods to achieve my goal.

Answer 1

If you want the actually used elements in r, use domain and range: dom(r) and ran(r). If you want all possible values, just use A resp. B.