Although we restrict our attention almost exclusively to functions in this course, the reality is that there are a lot of relations that are not functions. Any time that there is uncertainty for example, the relation is most probably not a function.

Although it is less frequent, it is indeed possible to have equations that do not represent functions. Most commonly these involve equality between variables that represent far more complex objects than numbers. Since we will be primarily focusing on functions we won’t be encountering these things in practice in this course, but it is important to remember that just because you see an equality does not automatically mean it is a function.