Functions:Forward Image

In mathematics, the image of a function is the set of all output values it may produce.

More generally, evaluating a given function $f$ at each element of a given subset $A$ of its domain produces a set, called the "image of $A$ under (or through) $f$". Similarly, the inverse image (or preimage) of a given subset $B$ of the codomain of $f$, is the set of all elements of the domain that map to the members of $B$.

Image and inverse image may also be defined for general binary relations, not just functions.

Definition

The word "image" is used in three related ways. In these definitions, $f:X\to \ Y$ is a function from the set $X$ to the set $Y$.

Image of an element

If $x$ is a member of $X$, then the image of $x$ under $f$, denoted $f(x)$, is the value of $f$ when applied to $x$. $f(x)$ is alternatively known as the output of $f$ for argument $x$.

Given $y$, the function $f$ is said to "take the value $y$" or "take $y$ as a value" if there exists some $x$ in the function's domain such that $f(x)=y$. Similarly, given a set $S$, $f$ is said to "take a value in $S$" if there exists some $x$ in the function's domain such that $f(x)\in S$. However, "$f$ takes all values in $S$" and "$f$ is valued in $S$" means that $f(x)\in S$ for every point $x$ in $f$'s domain.

Image of a subset

The image of a subset $A\subseteq X$ under $f$, denoted $f[A]$, is the subset of $Y$ which can be defined using set-builder notation as follows:

$f[A]=\{f(x):x\in A\}$

When there is no risk of confusion, $f[A]$ is simply written as $f(A)$. This convention is a common one; the intended meaning must be inferred from the context. This makes $f[\,\cdot \,]$ a function whose domain is the power set of $X$ (the set of all subsets of $X$), and whose codomain is the power set of $Y$.

Image of a function

The image of a function is the image of its entire domain, also known as the range of the function.[3] This usage should be avoided because the word "range" is also commonly used to mean the codomain of {\displaystyle f.}f.

Generalization to binary relations

If $R$ is an arbitrary binary relation on $X\times Y$, then the set $\{y\in Y:xRy{\text{ for some }}x\in X\}$ is called the image, or the range, of $R$. Dually, the set $\{x\in X:xRy{\text{ for some }}y\in Y\}$ is called the domain of $R$.

Examples

1. $f:\{1,2,3\}\to \{a,b,c,d\}$ defined by $f(x)=\left\{{\begin{matrix}a,&{\mbox{if }}x=1\\a,&{\mbox{if }}x=2\\c,&{\mbox{if }}x=3.\end{matrix}}\right.$

The image of the set $\{2,3\}$ under $f$ is $f(\{2,3\})=\{a,c\}.$ The image of the function $f$ is $\{a,c\}.$ The preimage of $a$ is $f^{-1}(\{a\})=\{1,2\}.$ The preimage of $\{a,b\}$ is also $f^{-1}(\{1,2\})=\{1,2\}.$ The preimage of $\{b,d\},$ is the empty set $\{\,\}=\varnothing .$

1. $f:\mathbb {R} \to \mathbb {R}$ defined by $f(x)=x^{2}.$

The image of $\{-2,3\}$ under $f$ is $f^{-1}(\{-2,3\})=\{4,9\},$ and the image of $f$ is $\mathbb {R} ^{+}$ (the set of all positive real numbers and zero). The preimage of $\{4,9\}$ under $f$ is $f^{-1}(\{4,9\})=\{-3,-2,2,3\}.$ The preimage of set $N=\{n\in \mathbb {R} :n<0\}$ under $f$ is the empty set, because the negative numbers do not have square roots in the set of reals.

1. $f:\mathbb {R} ^{2}\to \mathbb {R}$ defined by $f(x,y)=x^{2}+y^{2}.$

The fiber $f^{-1}(\{a\})$ are concentric circles]] about the origin, the origin itself, and the empty set, depending on whether $a>0,a=0,{\text{ or }}a<0,$ respectively. (if $a>0,$ then the fiber $f^{-1}(\{a\})$ is the set of all $(x,y)\in \mathbb {R} ^{2}$ satisfying the equation of the origin-concentric ring $x^{2}+y^{2}=a.$)

1. If $M$ is a manifold and $\pi :TM\to M$ is the canonical projection from the tangent bundle $TM$ to $M,$ then the fibers of $\pi$ are the tangent spaces $T_{x}(M){\text{ for }}x\in M.$ This is also an example of a fiber bundle.
2. A quotient group is a homomorphic image.