In mathematics, a function is a binary relation over two sets that associates to every element of the first set exactly one element of the second set. Typical examples are functions from integers to integers or from the real numbers to real numbers.
A function has only 1 y value for any given x value.
You can use the "vertical line test" to check if a graph is of a function. Move a vertical line (you can use the edge of a ruler) from left to right across the graph. If at any point the line intersects the curve in two or more places, the curve is not a function.
