Editing Graph of a function

{{Short description|Representation of a mathematical function}}
{{About||graph-theoretic representation of a function|Functional graph}}
{{more citations needed|date=August 2014}}
[[File:Polynomial of degree three.svg|class=skin-invert-image|thumb|250x250px|Graph of the function <math>f(x)=\frac{x^3+3x^2-6x-8}{4}.</math>]]
{{functions}}

In [[mathematics]], the '''graph of a function''' <math>f</math> is the set of [[ordered pair]]s <math>(x, y)</math>, where <math>f(x) = y.</math> In the common case where <math>x</math> and <math>f(x)</math> are [[real number]]s, these pairs are [[Cartesian coordinates]] of points in a [[plane (geometry)|plane]] and often form a [[Plane curve|curve]].
The graphical representation of the graph of a [[Function (mathematics)|function]] is also known as a ''[[Plot (graphics)|plot]]''.

In the case of [[Bivariate function|functions of two variables]] – that is, functions whose [[Domain of a function|domain]] consists of pairs <math>(x, y)</math> –, the graph usually refers to the set of [[ordered triple]]s <math>(x, y, z)</math> where <math>f(x,y) = z</math>. This is a subset of [[three-dimensional space]]; for a continuous [[real-valued function]] of two real variables, its graph forms a [[Surface (mathematics)|surface]], which can be visualized as a ''[[surface plot (graphics)|surface plot]]''.

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 coordinate system|rectangular axes]]; see ''[[Plot (graphics)]]'' for details.

{{anchor|graph of a relation}}A graph of a function is a special case of a [[Relation (mathematics)|relation]]. 
In the modern [[foundations of mathematics]], and, typically, in [[set theory]], a function is actually equal to its graph.<ref name="Pinter2014">{{cite book|author=Charles C Pinter|title=A Book of Set Theory|url=https://books.google.com/books?id=iUT_AwAAQBAJ&pg=PA49|year=2014|orig-year=1971|publisher=Dover Publications|isbn=978-0-486-79549-2|pages=49}}</ref> However, it is often useful to see functions as [[Map (mathematics)|mappings]],<ref>{{cite book|author=T. M. Apostol|authorlink=Tom M. Apostol|title=Mathematical Analysis|year=1981|publisher=Addison-Wesley|page=35}}</ref> which consist not only of the relation between input and output, but also which set is the domain, and which set is the [[codomain]]. For example, to say that a function is onto ([[Surjective function|surjective]]) or not the codomain should be taken into account. The graph of a function on its own does not determine the codomain. It is common<ref>{{cite book|author=P. R. Halmos|title=A Hilbert Space Problem Book|url=https://archive.org/details/hilbertspaceprob00halm_811|url-access=limited|year=1982|publisher=Springer-Verlag|isbn=0-387-90685-1|page=[https://archive.org/details/hilbertspaceprob00halm_811/page/n47 31]}}</ref> to use both terms ''function'' and ''graph of a function'' since even if considered the same object, they indicate viewing it from a different perspective.
[[File:X^4 - 4^x.PNG|class=skin-invert-image|350px|thumb|Graph of the function <math>f(x) = x^4 - 4^x</math> over the [[Interval (mathematics)|interval]] [−2,+3]. Also shown are the two real roots and the local minimum that are in the interval.]]

== Definition ==

Given a [[function (mathematics)|function]] <math>f : X \to Y</math> from a set {{mvar|X}} (the [[domain of a function|domain]]) to a set {{mvar|Y}} (the [[codomain]]), the graph of the function is the set<ref>{{cite book|author=D. S. Bridges|title=Foundations of Real and Abstract Analysis|url=https://archive.org/details/springer_10.1007-978-0-387-22620-0|year=1991|publisher=Springer|page=[https://archive.org/details/springer_10.1007-978-0-387-22620-0/page/n292 285]|isbn=0-387-98239-6}}</ref>  
<math display=block>G(f) = \{(x,f(x)) : x \in X\},</math>
which is a subset of the [[Cartesian product]] <math>X\times Y</math>. In the definition of a function in terms of [[set theory]], it is common to identify a function with its graph, although, formally, a function is formed by the triple consisting of its domain, its codomain and its graph.

== Examples ==

=== Functions of one variable ===

[[File:Three-dimensional graph.png|right|thumb|250px|Graph of the [[Function (mathematics)|function]] <math>f(x, y) = \sin\left(x^2\right) \cdot \cos\left(y^2\right).</math>]]

The graph of the function <math>f : \{1,2,3\} \to \{a,b,c,d\}</math> defined by
<math display=block>f(x)=
        \begin{cases}
              a, & \text{if }x=1, \\ d, & \text{if }x=2, \\ c, & \text{if }x=3, 
        \end{cases}
  </math>
is the subset of the set <math>\{1,2,3\} \times \{a,b,c,d\}</math>
<math display=block>G(f) = \{ (1,a), (2,d), (3,c) \}.</math>

From the graph, the domain <math>\{1,2,3\}</math> is recovered as the set of first component of each pair in the graph <math>\{1,2,3\} = \{x :\ \exists y,\text{ such that }(x,y) \in G(f)\}</math>.
Similarly, the [[Range of a function|range]] can be recovered as <math>\{a,c,d\} = \{y : \exists x,\text{ such that }(x,y)\in G(f)\}</math>.
The codomain <math>\{a,b,c,d\}</math>, however, cannot be determined from the graph alone.

The graph of the cubic polynomial on the [[real line]]
<math display=block>f(x) = x^3 - 9x</math>
is
<math display=block>\{ (x, x^3 - 9x) : x \text{ is a real number} \}.</math>

If this set is plotted on a [[Cartesian plane]], the result is a curve (see figure).
{{clear}}

=== Functions of two variables ===

[[File:F(x,y)=−((cosx)^2 + (cosy)^2)^2.PNG|class=skin-invert-image|thumb|250px|Plot of the graph of <math>f(x, y) = - \left(\cos\left(x^2\right) + \cos\left(y^2\right)\right)^2,</math> also showing its gradient projected on the bottom plane.]]

The graph of the [[trigonometric function]]
<math display=block>f(x,y) = \sin(x^2)\cos(y^2)</math>
is
<math display=block>\{ (x, y, \sin(x^2) \cos(y^2)) : x \text{ and } y \text{ are real numbers} \}.</math>

If this set is plotted on a [[Cartesian coordinate system#Cartesian coordinates in three dimensions|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:
<math display=block>f(x, y) = -(\cos(x^2) + \cos(y^2))^2.</math>

== See also ==
{{div col|colwidth=25em}}
* [[Asymptote]]
* [[Chart]]
* [[Plot (graphics)|Plot]]
* [[Concave function]]
* [[Convex function]]
* [[Contour line|Contour plot]]
* [[Critical point (mathematics)|Critical point]]
* [[Derivative]]
* [[Epigraph (mathematics)|Epigraph]]
* [[Normal (geometry)|Normal to a graph]]
* [[Slope]]
* [[Stationary point]]
* [[Tetraview]]
* [[Vertical translation]]
* [[y-intercept]]
{{div col end}}

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* {{Zălinescu Convex Analysis in General Vector Spaces 2002}} <!-- {{sfn|Zălinescu|2002|pp=}} -->
{{refend}}

== External links ==
{{Commons category|Function plots}}
* Weisstein, Eric W. "[http://mathworld.wolfram.com/FunctionGraph.html Function Graph]." From MathWorld—A Wolfram Web Resource.

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[[Category:Charts]]
[[Category:Functions and mappings]]
[[Category:Numerical function drawing]]