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In mathematics, the image of an element x in a set X under the function f : X → Y, denoted by f(x), is the unique y in Y that is associated with x. The image of a subset A ⊆ X under f is the subset of Y defined by
- f(A) = {y ∈ Y | y = f(x) for some x ∈ A}
Notice that the range of f is the image f(X) of its domain X.
With this definition, the image f becomes a function whose domain is the set of all subsets of X (also known as the power set of X) and whose codomain is the power set of Y. Note that the same notation is used for the original function f and its image. This is a common convention; the intended usage must be inferred by context.
The preimage or inverse image of a set B ⊆ Y under f is the subset of X defined by
- f −1(B) = {x ∈ X | f(x) ∈ B}
Note that f −1 should not be confused with the inverse function. The two only coincide if f is bijective. f −1 is a new function whose domain is the power set of Y and whose codomain is the power set of X.
Example
1. f: {1,2,3} → {a,b,c,d} defined by <math>f(x)=\left\{\begin{matrix} a, & \mbox{if }x=1 \\ d, & \mbox{if }x=2 \\ c, & \mbox{if }x=3. \end{matrix}\right.<math>
In this example, the image of {2,3} under f is f({2, 3}) = {c, d} and the range of f is {a, c, d}. The preimage of {a, b} is f −1({a, b}) = {1}.
2. f: R → R defined by f(x)=x2.
In this example, the image of [-2,3] under f is f([-2,3])=[0,9] and the range of f is the set of nonnegative real numbers. The preimage of [0,9] under f is f −1([0,9])=[-3,3].
Consequences
Some consequences that follow immediately from these definitions are:
- f(A1 ∪ A2) = f(A1) ∪ f(A2)
- f(A1 ∩ A2) ⊆ f(A1) ∩ f(A2)
- f −1(B1 ∪ B2) = f −1(B1) ∪ f −1(B2)
- f −1(B1 ∩ B2) = f −1(B1) ∩ f −1(B2)
- f(f −1(B)) ⊆ B
- f −1(f(A)) ⊇ A
- A1 ⊆ A2 implies f(A1) ⊆ f(A2)
- B1 ⊆ B2 implies f −1(B1) ⊆ f −1(B2)
These are valid for arbitrary subsets A, A1 and A2 of the domain and arbitrary subsets B, B1 and B2 of the codomain.
The results relating images and preimages to the algebra of intersection and union work for any collections of subsets, not just for pairs of subsets.
See also
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