Functions:Range - Revision history

Khanh at 21:13, 12 January 2022

2022-01-12T21:13:03Z

Khanh at 21:12, 12 January 2022

2022-01-12T21:12:19Z

Khanh: Created page with "In mathematics, the '''range of a function''' may refer to either of two closely related concepts: * The codomain of the function * The image of the function Given two sets..."

2022-01-12T21:11:54Z

Created page with "In mathematics, the '''range of a function''' may refer to either of two closely related concepts: * The codomain of the function * The image of the function Given two sets..."

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In mathematics, the '''range of a function''' may refer to either of two closely related concepts:

* The codomain of the function
* The image of the function

Given two sets {{mvar|X}} and {{mvar|Y}}, a [[binary relation]] {{mvar|f}} between {{mvar|X}} and {{mvar|Y}} is a (total) function (from {{mvar|X}} to {{mvar|Y}}) if for every {{mvar|x}} in {{mvar|X}} there is exactly one {{mvar|y}} in {{mvar|Y}} such that {{mvar|f}} relates {{mvar|x}} to {{mvar|y}}. The sets {{mvar|X}} and {{mvar|Y}} are called domain and codomain of {{mvar|f}}, respectively. The image of {{mvar|f}} is then the subset of {{mvar|Y}} consisting of only those elements {{mvar|y}} of {{mvar|Y}} such that there is at least one {{mvar|x}} in {{mvar|X}} with {{math|1=''f''(''x'') = ''y''}}.

==Terminology==
As the term "range" can have different meanings, it is considered a good practice to define it the first time it is used in a textbook or article. Older books, when they use the word "range", tend to use it to mean what is now called the codomain. More modern books, if they use the word "range" at all, generally use it to mean what is now called the image. To avoid any confusion, a number of modern books don't use the word "range" at all.<ref>Rudin 1991, page 99.</ref>

==Elaboration and example==
Given a function
:<math>f \colon X \to Y</math>
with domain <math>X</math>, the range of <math>f</math>, sometimes denoted <math>\operatorname{ran}(f)</math> or <math>\operatorname{Range}(f)</math>, may refer to the codomain or target set <math>Y</math> (i.e., the set into which all of the output of <math>f</math> is constrained to fall), or to <math>f(X)</math>, the image of the domain of <math>f</math> under <math>f</math> (i.e., the subset of <math>Y</math> consisting of all actual outputs of <math>f</math>). The image of a function is always a subset of the codomain of the function.

As an example of the two different usages, consider the function <math>f(x) = x^2</math> as it is used in real analysis (that is, as a function that inputs a real number and outputs its square). In this case, its codomain is the set of real numbers <math>\mathbb{R}</math>, but its image is the set of non-negative real numbers <math>\mathbb{R}^+</math>, since <math>x^2</math> is never negative if <math>x</math> is real. For this function, if we use "range" to mean ''codomain'', it refers to <math>\mathbb{{\displaystyle \mathbb {R} ^{}}}</math>; if we use "range" to mean ''image'', it refers to <math>\mathbb{R}^+</math>.

In many cases, the image and the codomain can coincide. For example, consider the function <math>f(x) = 2x</math>, which inputs a real number and outputs its double. For this function, the codomain and the image are the same (both being the set of real numbers), so the word range is unambiguous.

← Older revision		Revision as of 21:13, 12 January 2022
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	In many cases, the image and the codomain can coincide. For example, consider the function <math>f(x) = 2x</math>, which inputs a real number and outputs its double. For this function, the codomain and the image are the same (both being the set of real numbers), so the word range is unambiguous.		In many cases, the image and the codomain can coincide. For example, consider the function <math>f(x) = 2x</math>, which inputs a real number and outputs its double. For this function, the codomain and the image are the same (both being the set of real numbers), so the word range is unambiguous.
		+
		+	== Licensing ==
		+	Content obtained and/or adapted from:
		+	* [https://en.wikipedia.org/wiki/Range_of_a_function Range of a function, Wikipedia] under a CC BY-SA license

@@ Line 4: / Line 4: @@
 * The image of the function
-Given two sets {{mvar|X}} and {{mvar|Y}}, a [[binary relation]] {{mvar|f}} between {{mvar|X}} and {{mvar|Y}} is a (total) function (from {{mvar|X}} to {{mvar|Y}}) if for every {{mvar|x}} in {{mvar|X}} there is exactly one {{mvar|y}} in {{mvar|Y}} such that {{mvar|f}} relates {{mvar|x}} to {{mvar|y}}. The sets {{mvar|X}} and {{mvar|Y}} are called  domain and codomain of {{mvar|f}}, respectively. The image of {{mvar|f}} is then the subset of {{mvar|Y}} consisting of only those elements {{mvar|y}} of {{mvar|Y}} such that there is at least one {{mvar|x}} in {{mvar|X}} with {{math|1=''f''(''x'') = ''y''}}.
+Given two sets {{mvar|X}} and {{mvar|Y}}, a binary relation {{mvar|f}} between {{mvar|X}} and {{mvar|Y}} is a (total) function (from {{mvar|X}} to {{mvar|Y}}) if for every {{mvar|x}} in {{mvar|X}} there is exactly one {{mvar|y}} in {{mvar|Y}} such that {{mvar|f}} relates {{mvar|x}} to {{mvar|y}}. The sets {{mvar|X}} and {{mvar|Y}} are called  domain and codomain of {{mvar|f}}, respectively. The image of {{mvar|f}} is then the subset of {{mvar|Y}} consisting of only those elements {{mvar|y}} of {{mvar|Y}} such that there is at least one {{mvar|x}} in {{mvar|X}} with {{math|1=''f''(''x'') = ''y''}}.
 ==Terminology==
-As the term "range" can have different meanings, it is considered a good practice to define it the first time it is used in a textbook or article. Older books, when they use the word "range", tend to use it to mean what is now called the codomain.  More modern books, if they use the word "range" at all, generally use it to mean what is now called the image. To avoid any confusion, a number of modern books don't use the word "range" at all.<ref>Rudin 1991, page 99.</ref>
+As the term "range" can have different meanings, it is considered a good practice to define it the first time it is used in a textbook or article. Older books, when they use the word "range", tend to use it to mean what is now called the codomain.  More modern books, if they use the word "range" at all, generally use it to mean what is now called the image. To avoid any confusion, a number of modern books don't use the word "range" at all.
 ==Elaboration and example==