In this section we’ll talk about how to determine whether a graph represents a one-to-one function. If a relation is a function, then it has exactly one y-value for each x-value. If a function is one-to-one, it also has exactly one x-value for each y-value. The reason we care about one-to-one functions is because only a one-to-one function has an inverse. If the function is not one-to-one, then some restrictions might be needed on the domain of the function to make it invertible. The first way we’ll look at whether or not a function is one-to-one is using the Horizontal Line Test.

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