**Horizontal Line Test**

We use the **horizontal line test** to determine if the graph of a function is a one-to-one function. If any horizontal line intersects the graph in all possible places at exactly one point, then it is a graph of a one-to-one function.

In addition, if a function f(x) is a one-to-one function, then the function has an inverse function written as, {f^{ - 1}}\left( y \right).