when a question states that A is isomorphic to B, do we define the function from A to B or from B to A?
im quite confused about how we should define the function..
By "the function", I suppose you mean "the isomorphism"?
Usually, when we say , we mean that there exists an isomorphism . (There could be more than one.) But it doesn't really matter which direction we suppose the arrow to be going, because is an isomorphism . (Prove it!)