1. The problem statement, all variables and given/known data For the given relation, indicate if it is surjective, injective, both (bijective), or neither, and give a brief rationale for your answer. "The function that assigns to everybody in the US their social security number." 3. The attempt at a solution The wording on this, to me, isn't too clear, but my guess is that it means the Domain is possible Social Security numbers, and the Codomain is "everybody in the US." If you interpret it differently, please let me know. Not everybody in the US has a social security number, so it certainly is not surjective. It is injective, since each SS# is assigned to one and only one person. However, the thing that is throwing me off is, is it a function if there are elements in the domain that are not assigned to any element in the codomain? For example, the SS# 000-00-000 has never been used, so I'm not sure if that matters. So it is either "neither", since it is not a function, or "injective".