I am going on my own through a chapter on functions and here is something that puzzled me in the definition: Each element in the domain is paired with just one element in the range. I guess my calculus knowledge interferes, but what about function like f(x) = sqrt(x). It has two roots: + and -. How does set theory account for that? Or is sqrt(x) not a function in set-theoretic terms Although f(x) = x^2 fits the definition of the function. Could someone please explain? Thanks in advance.