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{{Functions}}
In [[mathematics]], an '''injective function''' (also known as '''injection''', or '''one-to-one function'''<ref>Sometimes ''one-one function'', in Indian mathematical education.
{{Cite web |title=Chapter 1:Relations and functions |url=https://ncert.nic.in/ncerts/l/lemh101.pdf}}</ref> ) is a [[function (mathematics)|function]] {{math|''f''}} that maps [[Distinct (mathematics)|distinct]] elements of its ___domain to distinct elements; that is, {{math|1=''x''<sub>1</sub> ≠ ''x''<sub>2</sub>}} implies {{math|''f''(''x''<sub>1</sub>) {{≠}} ''f''(''x''<sub>2</sub>)}}. (Equivalently, {{math|''f''(''x''<sub>1</sub>) {{=}} ''f''(''x''<sub>2</sub>)}} implies {{math|1=''x''<sub>1</sub> = ''x''<sub>2</sub>}} in the equivalent [[Contraposition|contrapositive]] statement.) In other words, every element of the function's [[codomain]] is the [[Image (mathematics)|image]] of {{em|at most}} one element of its [[Domain of a function|___domain]].<ref name=":0">{{Cite web|url=https://www.mathsisfun.com/sets/injective-surjective-bijective.html|title=Injective, Surjective and Bijective|website=www.mathsisfun.com|access-date=2019-12-07}}</ref> The term {{em|one-to-one function}} must not be confused with {{em|one-to-one correspondence}} that refers to [[bijective function]]s, which are functions such that each element in the codomain is an image of exactly one element in the ___domain. A [[homomorphism]] between [[algebraic structure]]s is a function that is compatible with the operations of the structures. For all common algebraic structures, and, in particular for [[vector space]]s, an {{em|injective homomorphism}} is also called a {{em|[[monomorphism]]}}. However, in the more general context of [[category theory]], the definition of a monomorphism differs from that of an injective homomorphism.<ref>{{Cite web|url=https://stacks.math.columbia.edu/tag/00V5|title=Section 7.3 (00V5): Injective and surjective maps of presheaves—The Stacks project|website=stacks.math.columbia.edu|access-date=2019-12-07}}</ref> This is thus a theorem that they are equivalent for algebraic structures; see {{slink|Homomorphism|Monomorphism}} for more details.
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==Gallery==
{{Gallery
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|Image:Injection.svg|An '''injective''' non-surjective function (injection, not a bijection)
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{{Gallery
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|Image:Non-injective function1.svg|Not an injective function. Here <math>X_1</math> and <math>X_2</math> are subsets of <math>X, Y_1</math> and <math>Y_2</math> are subsets of <math>Y</math>: for two regions where the function is not injective because more than one ___domain [[Element (mathematics)|element]] can map to a single range element. That is, it is possible for {{em|more than one}} <math>x</math> in <math>X</math> to map to the {{em|same}} <math>y</math> in <math>Y.</math>
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