Difference between revisions of "Functions:Forward Image"
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The word "image" is used in three related ways. In these definitions, <math>f:X\to\ Y</math> is a function from the set <math>X</math> to the set <math>Y</math>. | The word "image" is used in three related ways. In these definitions, <math>f:X\to\ Y</math> is a function from the set <math>X</math> to the set <math>Y</math>. | ||
| − | Image of an element | + | ===Image of an element=== |
| − | If | + | If <math>x</math> is a member of <math>X</math>, then the image of <math>x</math> under <math>f</math>, denoted <math>f(x)</math>, is the value of <math>f</math> when applied to <math>x</math>. <math>f(x)</math> is alternatively known as the output of <math>f</math> for argument <math>x</math>. |
| − | Given | + | Given <math>y</math>, the function <math>f</math> is said to "take the value <math>y</math>" or "take <math>y</math> as a value" if there exists some <math>x</math> in the function's domain such that <math>f(x)=y</math>. Similarly, given a set <math>S</math>, <math>f</math> is said to "take a value in <math>S</math>" if there exists some <math>x</math> in the function's domain such that <math>f(x)\in S</math>. However, "<math>f</math> takes all values in <math>S</math>" and "<math>f</math> is valued in <math>S</math>" means that <math>f(x)\in S</math> for every point <math>x</math> in <math>f</math>'s domain. |
| − | Image of a subset | + | ===Image of a subset=== |
The image of a subset {\displaystyle A\subseteq X}A\subseteq X under {\displaystyle f,}f, denoted {\displaystyle f[A],}{\displaystyle f[A],} is the subset of {\displaystyle Y}Y which can be defined using set-builder notation as follows:[1][2] | The image of a subset {\displaystyle A\subseteq X}A\subseteq X under {\displaystyle f,}f, denoted {\displaystyle f[A],}{\displaystyle f[A],} is the subset of {\displaystyle Y}Y which can be defined using set-builder notation as follows:[1][2] | ||
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When there is no risk of confusion, {\displaystyle f[A]}{\displaystyle f[A]} is simply written as {\displaystyle f(A).}{\displaystyle f(A).} This convention is a common one; the intended meaning must be inferred from the context. This makes {\displaystyle f[\,\cdot \,]}{\displaystyle f[\,\cdot \,]} a function whose domain is the power set of {\displaystyle X}X (the set of all subsets of {\displaystyle X}X), and whose codomain is the power set of {\displaystyle Y.}Y. See § Notation below for more. | When there is no risk of confusion, {\displaystyle f[A]}{\displaystyle f[A]} is simply written as {\displaystyle f(A).}{\displaystyle f(A).} This convention is a common one; the intended meaning must be inferred from the context. This makes {\displaystyle f[\,\cdot \,]}{\displaystyle f[\,\cdot \,]} a function whose domain is the power set of {\displaystyle X}X (the set of all subsets of {\displaystyle X}X), and whose codomain is the power set of {\displaystyle Y.}Y. See § Notation below for more. | ||
| − | Image of a function | + | ===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. | 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 | + | ===Generalization to binary relations=== |
If {\displaystyle R}R is an arbitrary binary relation on {\displaystyle X\times Y,}{\displaystyle X\times Y,} then the set {\displaystyle \{y\in Y:xRy{\text{ for some }}x\in X\}}{\displaystyle \{y\in Y:xRy{\text{ for some }}x\in X\}} is called the image, or the range, of {\displaystyle R.}R. Dually, the set {\displaystyle \{x\in X:xRy{\text{ for some }}y\in Y\}}{\displaystyle \{x\in X:xRy{\text{ for some }}y\in Y\}} is called the domain of {\displaystyle R.}R. | If {\displaystyle R}R is an arbitrary binary relation on {\displaystyle X\times Y,}{\displaystyle X\times Y,} then the set {\displaystyle \{y\in Y:xRy{\text{ for some }}x\in X\}}{\displaystyle \{y\in Y:xRy{\text{ for some }}x\in X\}} is called the image, or the range, of {\displaystyle R.}R. Dually, the set {\displaystyle \{x\in X:xRy{\text{ for some }}y\in Y\}}{\displaystyle \{x\in X:xRy{\text{ for some }}y\in Y\}} is called the domain of {\displaystyle R.}R. | ||
Revision as of 09:27, 12 October 2021
In mathematics, the image of a function is the set of all output values it may produce.
More generally, evaluating a given function Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} at each element of a given subset Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A} of its domain produces a set, called the "image of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A} under (or through) Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} ". Similarly, the inverse image (or preimage) of a given subset Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle B} of the codomain of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} , is the set of all elements of the domain that map to the members of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle B} .
Image and inverse image may also be defined for general binary relations, not just functions.
Contents
Definition
The word "image" is used in three related ways. In these definitions, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f:X\to\ Y} is a function from the set Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X} to the set Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle Y} .
Image of an element
If Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} is a member of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle X} , then the image of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} under Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} , denoted Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(x)} , is the value of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} when applied to Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} . Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(x)} is alternatively known as the output of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} for argument Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} .
Given Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle y} , the function Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} is said to "take the value Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle y} " or "take Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle y} as a value" if there exists some Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} in the function's domain such that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(x)=y} . Similarly, given a set Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S} , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} is said to "take a value in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S} " if there exists some Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} in the function's domain such that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(x)\in S} . However, "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} takes all values in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S} " and "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} is valued in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle S} " means that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(x)\in S} for every point Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f} 's domain.
Image of a subset
The image of a subset {\displaystyle A\subseteq X}A\subseteq X under {\displaystyle f,}f, denoted {\displaystyle f[A],}{\displaystyle f[A],} is the subset of {\displaystyle Y}Y which can be defined using set-builder notation as follows:[1][2]
{\displaystyle f[A]=\{f(x):x\in A\}}{\displaystyle f[A]=\{f(x):x\in A\}} When there is no risk of confusion, {\displaystyle f[A]}{\displaystyle f[A]} is simply written as {\displaystyle f(A).}{\displaystyle f(A).} This convention is a common one; the intended meaning must be inferred from the context. This makes {\displaystyle f[\,\cdot \,]}{\displaystyle f[\,\cdot \,]} a function whose domain is the power set of {\displaystyle X}X (the set of all subsets of {\displaystyle X}X), and whose codomain is the power set of {\displaystyle Y.}Y. See § Notation below for more.
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 {\displaystyle R}R is an arbitrary binary relation on {\displaystyle X\times Y,}{\displaystyle X\times Y,} then the set {\displaystyle \{y\in Y:xRy{\text{ for some }}x\in X\}}{\displaystyle \{y\in Y:xRy{\text{ for some }}x\in X\}} is called the image, or the range, of {\displaystyle R.}R. Dually, the set {\displaystyle \{x\in X:xRy{\text{ for some }}y\in Y\}}{\displaystyle \{x\in X:xRy{\text{ for some }}y\in Y\}} is called the domain of {\displaystyle R.}R.
