> "Definition 2-1.1. A function f is a set of ordered pairs, no two of > which have the same first element. The set of first elements of the > pairs is called the domain of the function, whereas the set of second > elements of the pairs is called the range. The domain and range > elements are related by a given rule"
Alas and alack, I forgot the citation:
Introduction to Vector and Tensor Analysis, by Robert C. Wrede.