What is the green line test?

The Green Line Test is a method used to determine whether a mapping of points in the plane (or a region in space) to another set of points is a function.

To perform the Green Line Test, you draw a horizontal or vertical line across the graph of the mapping. If the line intersects the graph no more than once at any point, then the mapping is a function. If the line intersects the graph more than once at any point, then the mapping is not a function.

The Green Line Test is a visual way to quickly identify if a given mapping is a function, and it is especially useful when working with graphs and visual representations. It is named after mathematician George David Green, who first introduced the concept.