## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

As depicted in the graph, there is one $y$-value for every $x-value$. In other words, the graph passes the Vertical Line Test. Thus, it is a function. The Horizontal Line Test states that any horizontal line should intersect the graph at no more than one point. Otherwise, the graph is not one-to-one. The given graph fails the Horizontal Line Test because some y-values appear not-uniquely. Thus, the function is not one-to-one.