When every element in the domain corresponds to exactly one element in the range, the relation is a function. A function passes the vertical line test: no vertical line passes through more than one point in the relation. The unit circle is thus not a function. The half-circle above the axis is the function .

The horizontal line test, which tests if any horizontal line intersects a graph at more than one point, can have three different results when applied to functions:

1. If no horizontal line intersects the function in more than one point, the function is one-to-one (or injective).

2. If every horizontal line intersects the function in at least one point, it is onto (or surjective).

3. If every horizontal line intersects the function in exactly one point, it is one-to-one and onto (or bijective).

Suppose is a function with domain and range . The inverse of is a function with domain and range such that if and only if . For the inverse to exist, the original function must be one-to-one and onto.

Let be the set of real numbers. The inverse of with domain and range is the function with the same domain and range.

Many functions that come up in practice are either not one-to-one or not onto. For example, because trigonometric functions are periodic they are many-to-one on . Also, except for the tangent and arctangent functions, the trig functions are not onto. For example, there is no real number such that .

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