Functions are a correspondence between two sets, called the **domain** and the **range**. When defining a function, you usually state what kind of numbers the domain (*x*) and range (*f(x)*) values can be. But even if you say they are real numbers, that does not mean that *all* real numbers can be used for *x*. It also does not mean that all real numbers can be function values, *f*(*x*). There may be restrictions on the domain and range. The restrictions partly depend on the *type* of function.

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