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| In [[mathematics]], the '''horizontal line test''' is a test used to determine whether a [[function (mathematics)|function]] is [[injective]] (i.e., one-to-one).<ref name="Stewart">{{cite book|last=Stewart|first=James|title=Single Variable Calculus: Early Transcendentals|year=2003|publisher=Brook/Cole|location=Toronto ON|isbn=0-534-39330-6|pages=64|url=http://www.stewartcalculus.com/media/8_home.php|edition=5th.|authorlink=James Stewart (mathematician)|accessdate=15 July 2012|quote=Therefore, we have the following geometric method for determining whether a function is one-to-one.}}</ref>
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| ==In calculus==
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| A ''horizontal line'' is a straight, flat line that goes from left to right. Given a function <math>f \colon \mathbb{R} \to \mathbb{R}</math> (i.e. from the [[real numbers]] to the real numbers), we can decide if it is [[injective]] by looking at horizontal lines that intersect the function's [[graph of a function|graph]]. If any horizontal line <math>y=c</math> intersects the graph in more than one point, the function is not injective. To see this, note that the points of intersection have the same y-value (because they lie on the line <math>y=c</math>) but different x values, which by definition means the function cannot be injective.<ref name="Stewart"/>
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| {| border="1"
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| | align="center"|[[Image:Horizontal-test-ok.png]]<br>
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| Passes the test (injective)
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| | align="center"|[[Image:Horizontal-test-fail.png]]<br>
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| Fails the test (not injective)
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| |}
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| Variations of the horizontal line test can be used to determine whether a function is [[surjective]] or [[bijective]]:
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| *The function ''f'' is [[surjective]] (i.e., onto) [[if and only if]] its graph intersects any horizontal line at '''least''' once.
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| *''f'' is bijective [[if and only if]] any horizontal line will intersect the graph '''exactly''' once.
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| ==In set theory==
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| Consider a function <math>f \colon X \to Y</math> with its corresponding [[graph of a function|graph]] as a subset of the [[Cartesian product]] <math>X \times Y</math>. Consider the horizontal lines in <math>X \times Y</math> :<math>\{(x,y_0) \in X \times Y: y_0 \text{ is constant}\} = X \times \{y_0\}</math>. The function ''f'' is [[injective]] [[if and only if]] each horizontal line intersects the graph at most once. In this case the graph is said to pass the horizontal line test. If any horizontal line intersects the graph more than once, the function fails the horizontal line test and is not injective.<ref>{{cite book|last=Zorn|first=Arnold Ostebee, Paul|title=Calculus from graphical, numerical, and symbolic points of view|year=2002|publisher=Brooks/Cole/Thomson Learning|location=Australia|isbn=0-03-025681-X|pages=185|url=http://books.google.com/books?id=D48RplvmxVUC&q=horizontal+line+test#search_anchor|edition=2nd ed.|quote=No horizontal line crosses the f-graph more than once.}}</ref>
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| == See also ==
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| *[[Vertical line test]]
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| *[[Function (mathematics)]]
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| *[[Inverse (mathematics)]]
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| ==References==
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| {{reflist}}
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| [[Category:Basic concepts in set theory]]
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| {{mathematics-stub}}
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