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### Image (mathematics)

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{{short description|The set of all values of a function}}(File:Codomain2.SVG|thumb|upright=1.5|f is a function from domain X to codomain Y. The yellow oval inside Y is the image of f.){{Group theory sidebar |Basics}}In mathematics, an image is the subset of a function's codomain which is the output of the function from a subset of its domain.Evaluating a function at each element of a subset X of the domain produces a set called the image of X under or through the function. The inverse image or preimage of a particular subset S of the codomain of a function is the set of all elements of the domain that map to the members of S.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 â†’ Y is a function from the set X to the set Y.

### Image of an element

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

### Image of a subset

The image of a subset A âŠ† X under f is the subset f[A] âŠ† Y defined by (using set-builder notation):
f[A] = { f(x) mid 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[.] 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. See Notation below.

### Image of a function

The image of a function is the image of its entire domain.

### Generalization to binary relations

If R is an arbitrary binary relation on XÃ—Y, the set { yâˆˆY | xRy for some xâˆˆX } is called the image, or the range, of R. Dually, the set { xâˆˆX | xRy for some yâˆˆY } is called the domain of R.

## Inverse image

{{Redirect|Preimage|the cryptographic attack on hash functions|preimage attack}}Let f be a function from X to Y. The preimage or inverse image of a set B âŠ† Y under f is the subset of X defined by
f^{-1}[ B ] = { x in X , | , f(x) in B }
The inverse image of a singleton, denoted by f âˆ’1[{y}] or by f âˆ’1[y], is also called the fiber over y or the level set of y. The set of all the fibers over the elements of Y is a family of sets indexed by Y.For example, for the function f(x) = x2, the inverse image of {4} would be {âˆ’2, 2}. Again, if there is no risk of confusion, denote f âˆ’1[B] by f âˆ’1(B), and think of f âˆ’1 as a function from the power set of Y to the power set of X. The notation f âˆ’1 should not be confused with that for inverse function. The notation coincides with the usual one, though, for bijections, in the sense that the inverse image of B under f is the image of B under f âˆ’1.

## Notation for image and inverse image

The traditional notations used in the previous section can be confusing. An alternativeBlyth 2005, p. 5 is to give explicit names for the image and preimage as functions between powersets:

### Arrow notation

• f^rightarrow:mathcal{P}(X)rightarrowmathcal{P}(Y) with f^rightarrow(A) = { f(a);|; a in A}
• f^leftarrow:mathcal{P}(Y)rightarrowmathcal{P}(X) with f^leftarrow(B) = { a in X ;|; f(a) in B}

### Star notation

• f_star:mathcal{P}(X)rightarrowmathcal{P}(Y) instead of f^rightarrow
• f^star:mathcal{P}(Y)rightarrowmathcal{P}(X) instead of f^leftarrow

### Other terminology

• An alternative notation for f[A] used in mathematical logic and set theory is f "A.BOOK, Set Theory for the Mathematician, Jean E. Rubin, Jean E. Rubin, xix, 1967, Holden-Day, B0006BQH7S, M. Randall Holmes: Inhomogeneity of the urelements in the usual models of NFU, December 29, 2005, on: Semantic Scholar, p. 2
• Some texts refer to the image of f as the range of f, but this usage should be avoided because the word "range" is also commonly used to mean the codomain of f.

## Examples

1. f: {1, 2, 3} â†’ {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.
{{paragraph break}} 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 âˆ’1({a}) = {1, 2}. The preimage of {a, b} is also {1, 2}. The preimage of {b, d} is the empty set {}.
1. f: R â†’ R defined by f(x) = x2.{{paragraph break}} The image of {âˆ’2, 3} under f is f({âˆ’2, 3}) = {4, 9}, and the image of f is R+. The preimage of {4, 9} under f is f âˆ’1({4, 9}) = {âˆ’3, âˆ’2, 2, 3}. The preimage of set N = {n âˆˆ R | n < 0} under f is the empty set, because the negative numbers do not have square roots in the set of reals.
2. f: R2 â†’ R defined by f(x, y) = x2 + y2.{{paragraph break}} The fibres f âˆ’1({a}) are concentric circles about the origin, the origin itself, and the empty set, depending on whether a > 0, a = 0, or a < 0, respectively.
3. If M is a manifold and Ï€: TM â†’ M is the canonical projection from the tangent bundle TM to M, then the fibres of Ï€ are the tangent spaces Tx(M) for xâˆˆM. This is also an example of a fiber bundle.
4. A quotient group is a homomorphic image.

## Properties

For every function f : X â†’ Y, all subsets A, A1, and A2 of X and all subsets B, B1, and B2 of Y, the following properties hold:
• f(A1 âˆª A2) = f(A1) âˆª f(A2)Kelley (1985), [{{Google books|plainurl=y|id=-goleb9Ov3oC|page=85|text=The image of the union of a family of subsets of X is the union of the images, but, in general, the image of the intersection is not the intersection of the images}} p. 85]
• f(A1 âˆ© A2) âŠ† f(A1) âˆ© f(A2)
• f(A âˆ© f âˆ’1(B)) = f(A) âˆ© B
• f âˆ’1(B1 âˆª B2) = f âˆ’1(B1) âˆª f âˆ’1(B2)
• f âˆ’1(B1 âˆ© B2) = f âˆ’1(B1) âˆ© f âˆ’1(B2)
• f(A) = âˆ… â‡” A = âˆ…
• f âˆ’1(B) = âˆ… â‡” B âŠ† (f(X))C
• f(A) âˆ© B = âˆ… â‡” A âˆ© f âˆ’1(B) = âˆ…
• f(A) âŠ† B â‡” A âŠ†  f âˆ’1(B)
• B âŠ† f(A) â‡” existsC âŠ† A (f(C) = B)
• f(f âˆ’1(B)) âŠ† BEquality holds if B is a subset of Im(f) or, in particular, if f is surjective. See Munkres, J.. Topology (2000), p. 19.
• f âˆ’1(f(A)) âŠ‡ AEquality holds if f is injective. See Munkres, J.. Topology (2000), p. 19.
• f(f âˆ’1(B)) = B âˆ© f(X)
• f âˆ’1(f(X)) = X
• A1 âŠ† A2 â‡’ f(A1) âŠ† f(A2)
• B1 âŠ† B2 â‡’ f âˆ’1(B1) âŠ† f âˆ’1(B2)
• f âˆ’1(BC) = (f âˆ’1(B))C
• (f |A)âˆ’1(B) = A âˆ© f âˆ’1(B).
The results relating images and preimages to the (Boolean) algebra of intersection and union work for any collection of subsets, not just for pairs of subsets:
• fleft(bigcup_{sin S}A_sright) = bigcup_{sin S} f(A_s)
• fleft(bigcap_{sin S}A_sright) subseteq bigcap_{sin S} f(A_s)
• f^{-1}left(bigcup_{sin S}B_sright) = bigcup_{sin S} f^{-1}(B_s)
• f^{-1}left(bigcap_{sin S}B_sright) = bigcap_{sin S} f^{-1}(B_s)
(Here, S can be infinite, even uncountably infinite.)With respect to the algebra of subsets, by the above the inverse image function is a lattice homomorphism while the image function is only a semilattice homomorphism (it does not always preserve intersections).

{{reflist}}

## References

• BOOK, Michael Artin, Artin, Michael, Algebra, 1991, Prentice Hall, 81-203-0871-9, {{inconsistent citations, }}
• T.S. Blyth, Lattices and Ordered Algebraic Structures, Springer, 2005, {{ISBN|1-85233-905-5}}.
• BOOK, Munkres, James R., Topology, 2, 2000, Prentice Hall, 978-0-13-181629-9,
• BOOK, Kelley, John L., General Topology, 2, Graduate Texts in Mathematics, 27, 1985, BirkhÃ¤user, 978-0-387-90125-1,
{{PlanetMath attribution|id=3276|title=Fibre}}

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