Editing Monomorphism

{{Short description|Injective homomorphism}}
{{about|the mathematical term|other uses|Monomorphic (disambiguation)|and|Polymorphism (disambiguation)}}
[[Image:Monomorphism scenarios.svg|right|thumb|220px]]

In the context of [[abstract algebra]] or [[universal algebra]], a '''monomorphism''' is an [[Injective function|injective]] [[homomorphism]]. A monomorphism from {{mvar|X}} to {{mvar|Y}} is often denoted with the notation  <math>X\hookrightarrow Y</math>.

In the more general setting of [[category theory]], a '''monomorphism''' (also called a '''monic morphism''' or a '''mono''') is a [[left-cancellative]] [[morphism]]. That is, an arrow {{math|''f'' : ''X'' → ''Y''}} such that for all objects {{math|''Z''}} and all morphisms {{math|''g''<sub>1</sub>, ''g''<sub>2</sub>: ''Z'' → ''X''}},
: <math>f \circ g_1 = f \circ g_2 \implies g_1 = g_2.</math>[[File:Monomorphism_pullback_square.png|thumb|225x225px|pullback of monomorphism with itself]]

Monomorphisms are a categorical generalization of [[injective function]]s (also called "one-to-one functions"); in some categories the notions coincide, but monomorphisms are more general, as in the [[#Examples|examples below]].

In the setting of [[Partially ordered set|posets]] intersections are [[Idempotence|idempotent]]: the intersection of anything with itself is itself. Monomorphisms generalize this property to arbitrary categories. A morphism is a monomorphism if it is idempotent with respect to [[Pullback (category theory)|pullbacks]].

The [[categorical dual]] of a monomorphism is an [[epimorphism]], that is, a monomorphism in a category ''C'' is an epimorphism in the [[dual category]] ''C''<sup>op</sup>. Every [[section (category theory)|section]] is a monomorphism, and every [[retract (category theory)|retraction]] is an epimorphism.

==Relation to invertibility==
Left-invertible morphisms are necessarily monic: if ''l'' is a left inverse for ''f'' (meaning ''l'' is a morphism and <math>l \circ f = \operatorname{id}_{X}</math>), then ''f'' is monic, as
: <math>f \circ g_1 = f \circ g_2 \Rightarrow l\circ f\circ g_1 = l\circ f\circ g_2 \Rightarrow g_1 = g_2.</math>

A left-invertible morphism is called a '''[[Section (category theory)|split mono]]''' or a '''section'''. 

However, a monomorphism need not be left-invertible. For example, in the category '''Group''' of all [[Group (mathematics)|groups]] and [[group homomorphism]]s among them, if ''H'' is a subgroup of ''G'' then the inclusion {{nowrap|''f'' : ''H'' → ''G''}} is always a monomorphism; but ''f'' has a left inverse in the category if and only if ''H'' has a [[Complement (group theory)|normal complement]] in ''G''. 

A morphism {{nowrap|''f'' : ''X'' → ''Y''}} is monic if and only if the induced map {{nowrap|''f''<sub>∗</sub> : Hom(''Z'', ''X'') → Hom(''Z'', ''Y'')}}, defined by {{nowrap|1=''f''<sub>∗</sub>(''h'') = ''f'' ∘ ''h''}} for all morphisms {{nowrap|''h'' : ''Z'' → ''X''}}, is [[injective]] for all objects ''Z''.

==Examples==
Every morphism in a [[concrete category]] whose underlying [[function (mathematics)|function]] is injective is a monomorphism; in other words, if morphisms are actually functions between sets, then any morphism which is a one-to-one function will necessarily be a monomorphism in the categorical sense.  In the [[category of sets]] the converse also holds, so the monomorphisms are exactly the [[injective]] morphisms.  The converse also holds in most naturally occurring categories of algebras because of the existence of a [[free object]] on one generator.  In particular, it is true in the categories of all groups, of all [[ring (mathematics)|ring]]s, and in any [[abelian category]].

It is not true in general, however, that all monomorphisms must be injective in other categories; that is, there are settings in which the morphisms are functions between sets, but one can have a function that is not injective and yet is a monomorphism in the categorical sense. For example, in the category '''Div''' of [[divisible group|divisible]] [[abelian group|(abelian) group]]s and [[group homomorphism]]s between them there are monomorphisms that are not injective: consider, for example, the quotient map {{nowrap|''q'' : '''Q''' → '''Q'''/'''Z'''}}, where '''Q''' is the rationals under addition, '''Z''' the integers (also considered a group under addition), and '''Q'''/'''Z''' is the corresponding [[quotient group]].  This is not an injective map, as for example every integer is mapped to 0. Nevertheless, it is a monomorphism in this category.  This follows from the implication {{nowrap|1=''q'' ∘ ''h'' = 0 ⇒ ''h'' = 0}}, which we will now prove.  If {{nowrap|''h'' : ''G'' → '''Q'''}}, where ''G'' is some divisible group, and {{nowrap|1=''q'' ∘ ''h'' = 0}}, then {{nowrap|''h''(''x'') ∈ '''Z''', ∀ ''x'' ∈ ''G''}}.  Now fix some {{nowrap|''x'' ∈ ''G''}}.  Without loss of generality, we may assume that {{nowrap|''h''(''x'') ≥ 0}} (otherwise, choose −''x'' instead).  Then, letting {{nowrap|1=''n'' = ''h''(''x'') + 1}}, since ''G'' is a divisible group, there exists some {{nowrap|''y'' ∈ ''G''}} such that {{nowrap|1=''x'' = ''ny''}}, so {{nowrap|1=''h''(''x'') = ''n'' ''h''(''y'')}}.  From this, and {{nowrap|1=0 ≤ ''h''(''x'') < ''h''(''x'') + 1 = ''n''}}, it follows that

:<math>0 \leq \frac{h(x)}{h(x) + 1} = h(y) < 1 </math>

Since {{nowrap|''h''(''y'') ∈ '''Z'''}}, it follows that {{nowrap|1=''h''(''y'') = 0}}, and thus {{nowrap|1=''h''(''x'') = 0 = ''h''(−''x''), ∀ ''x'' ∈ ''G''}}.  This says that {{nowrap|1=''h'' = 0}}, as desired.  

To go from that implication to the fact that ''q'' is a monomorphism, assume that {{nowrap|1=''q''  ∘ ''f'' = ''q'' ∘ ''g''}} for some morphisms {{nowrap|''f'', ''g'' : ''G'' → '''Q'''}}, where ''G'' is some divisible group. Then {{nowrap|1=''q'' ∘ (''f'' − ''g'') = 0}}, where {{nowrap|(''f'' − ''g'') : ''x'' ↦ ''f''(''x'') − ''g''(''x'')}}.  (Since {{nowrap|1=(''f'' − ''g'')(0) = 0}}, and {{nowrap|1=(''f'' − ''g'')(''x'' + ''y'') = (''f'' − ''g'')(''x'') + (''f'' − ''g'')(''y'')}}, it follows that {{nowrap|(''f'' − ''g'') ∈ Hom(''G'', '''Q''')}}).  From the implication just proved, {{nowrap|1=''q''  ∘ (''f'' − ''g'') = 0  ⇒ ''f'' − ''g'' = 0  ⇔ ∀ ''x'' ∈ ''G'', ''f''(''x'') = ''g''(''x'') ⇔ ''f'' = ''g''}}.  Hence ''q'' is a monomorphism, as claimed.

==Properties==
*In a [[topos]], every mono is an equalizer, and any map that is both monic and [[epic morphism|epic]] is an [[isomorphism (category theory)|isomorphism]].
*Every isomorphism is monic.

==Related concepts==
There are also useful concepts of ''regular monomorphism'', ''extremal monomorphism'', ''immediate monomorphism'', ''strong monomorphism'', and ''split monomorphism''. 

* A monomorphism is said to be '''regular''' if it is an [[equaliser (mathematics)|equalizer]] of some pair of parallel morphisms. 
* A monomorphism <math>\mu</math> is said to be '''extremal'''{{sfn|Borceux|1994}} if in each representation <math>\mu=\varphi\circ\varepsilon</math>, where <math>\varepsilon</math> is an epimorphism, the morphism <math>\varepsilon</math> is automatically an [[isomorphism]]. 
* A monomorphism <math>\mu</math> is said to be '''immediate''' if in each representation <math>\mu=\mu'\circ\varepsilon</math>, where <math>\mu'</math> is a monomorphism and <math>\varepsilon</math> is an epimorphism, the morphism <math>\varepsilon</math> is automatically an [[isomorphism]].
* [[File:Diagram-orthogonality-2.jpg|thumb]] A monomorphism <math>\mu:C\to D</math> is said to be '''strong'''{{sfn|Borceux|1994}}{{sfn|Tsalenko|Shulgeifer|1974}} if for any epimorphism <math>\varepsilon:A\to B</math> and any morphisms <math>\alpha:A\to C</math> and <math>\beta:B\to D</math> such that <math>\beta\circ\varepsilon=\mu\circ\alpha</math>, there exists a morphism <math>\delta:B\to C</math> such that <math>\delta\circ\varepsilon=\alpha</math> and <math>\mu\circ\delta=\beta</math>.
* A monomorphism <math>\mu</math> is said to be '''split''' if there exists a morphism <math>\varepsilon</math> such that <math>\varepsilon\circ\mu=1</math> (in this case <math>\varepsilon</math> is called a left-sided inverse for <math>\mu</math>).

==Terminology==

The companion terms ''monomorphism'' and ''epimorphism'' were originally introduced by [[Nicolas Bourbaki]]; Bourbaki uses ''monomorphism'' as shorthand for an injective function. Early category theorists believed that the correct generalization of injectivity to the context of categories was the cancellation property given above.  While this is not exactly true for monic maps, it is very close, so this has caused little trouble, unlike the case of epimorphisms.  [[Saunders Mac Lane]] attempted to make a distinction between what he called ''monomorphisms'', which were maps in a concrete category whose underlying maps of sets were injective, and ''monic maps'', which are monomorphisms in the categorical sense of the word.  This distinction never came into general use.

Another name for monomorphism is ''[[extension (model theory)|extension]]'', although this has other uses too.

==See also==
*[[Embedding]]
*[[Nodal decomposition]]
*[[Subobject]]

==Notes==
{{reflist}}

==References==
*{{cite book|last=Bergman|first=George|title=An Invitation to General Algebra and Universal Constructions |url=http://math.berkeley.edu/~gbergman/245/index.html |year=2015|publisher=Springer|isbn=978-3-319-11478-1}}
*{{cite book|last=Borceux|first=Francis|title=Handbook of Categorical Algebra. Volume 1: Basic Category Theory|year=1994|publisher=Cambridge University Press|isbn=978-0521061193}}
*{{springer|title=Monomorphism|id=p/m064800}}
*{{cite journal|last=Van Oosten|first=Jaap|title=Basic Category Theory|journal=Brics Lecture Series|url=http://www.math.uu.nl/people/jvoosten/syllabi/catsmoeder.pdf |year=1995|publisher=BRICS, Computer Science Department, University of Aarhus|issn=1395-2048}}
*{{cite book|last1=Tsalenko|first1=M.S.| last2=Shulgeifer|first2=E.G.|title=Foundations of category theory|year=1974|publisher=Nauka|isbn=5-02-014427-4}}

==External links==

*{{nlab|id=monomorphism}}
*{{nlab|id=strong+monomorphism|title=Strong monomorphism}}

[[Category:Morphisms]]
[[Category:Algebraic properties of elements]]