In mathematics, the graph of a function f is, formally, the set of all ordered pairs , and, in practice, the graphical representation of this set. If the function input x is a real number, the graph is a two-dimensional graph, and, for a continuous function, is a curve. If the function input x is an ordered pair of real numbers, the graph is the collection of all ordered triples , and for a continuous function is a surface.
The concept of the graph of a function is generalized to the graph of a relation. To test whether a graph of a relation represents a function of the first variable x, one uses the vertical line test. To test whether a graph represents a function of the second variable y, one uses the horizontal line test. If the function has an inverse, the graph of the inverse can be found by reflecting the graph of the original function over the line .
In science, engineering, technology, finance, and other areas, graphs are tools used for many purposes. In the simplest case one variable is plotted as a function of another, typically using rectangular axes; see Plot (graphics) for details.
In the modern foundations of mathematics, and, typically, in set theory, a function and its graph are the same thing. However, the concept of function is aimed to formalize the intuitive notion of "mapping", which is not directly apparent on the graph. Therefore, reasoning about functions is generally easier if a function is distinguished from its graphs, and if the graph is viewed as a graphical representation of the function.
The graph of the function.
The graph of the cubic polynomial on the real line
If this set is plotted on a Cartesian plane, the result is a curve (see figure).
The graph of the trigonometric function
If this set is plotted on a three dimensional Cartesian coordinate system, the result is a surface (see figure).
Oftentimes it is helpful to show with the graph, the gradient of the function and several level curves. The level curves can be mapped on the function surface or can be projected on the bottom plane. The second figure shows such a drawing of the graph of the function:
Given a function f of n variables: , the normal to the graph is
(up to multiplication by a constant). This is seen by considering the graph as a level set of the function , and using that is normal to the level sets.
The graph of a function is contained in a Cartesian product of sets. An X-Y plane is a cartesian product of two lines, called X and Y, while a cylinder is a cartesian product of a line and a circle, whose height, radius, and angle assign precise locations of the points. Fibre bundles are not Cartesian products, but appear to be up close. There is a corresponding notion of a graph on a fibre bundle called a section.