21. The “Vertical Line Test” from calculus says that a curve in the xy-plane is the graph
of a function of x if and only if no vertical line intersects the curve more than once.
Explain why this agrees with Definition 2.1.1.
Solution: We assume that the x-axis is the domain and the y-axis is the codomain of
the function that is to be defined by the given curve. According to Definition 2.1.1,
a subset of the plane defines a function if for each element x in the domain there
is a unique element y in the codomain such that (x, y) belongs to the subset of the
plane. If a vertical line intersects the curve in two distinct points, then there will be
points (x1, y1) and (x2, y2) on the curve with x1 = x2 and y1 6= y2. Thus if we apply
Definition 2.1.1 to the given curve, the uniqueness part of the definition translates
directly into the “vertical line test”.