Passing the horizontal line test means an exponential function must have an:

1. A one-to-one function never repeats an output. 2. The horizontal line test is how that is checked on a graph. 3. This means the exponential function will have an inverse. 4. That inverse turns out to be the logarithm. _Source: OpenStax Intermediate Algebra 2e (CC BY 4.0), Ch 10, sections 10.1-10.2 'Evaluate and Graph Exponential Functions'_