## Vertical line test for functions and relation

A relation is a function if no pair of the relation has the same first elements. On the graph,two points will lie in the same vertical line if and only if the first coordinate of the two points are same. Therefore if a vertical line intesect the Relation graph at two points then it is not a function. This is called the Vertical line test. If all vertical lines intersect a curve at most once then the curve represents a function