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| {{technical|date=January 2014}}
| | Hi there! :) My name is Lin, I'm a student studying Anthropology and Sociology from Syracuse, United States.<br><br>Feel free to visit my website ... [http://Www.mindspoke.net/members/latostarver/activity/421152/ Fifa 15 Coin generator] |
| In [[mathematics]], specifically in [[category theory]], the '''Yoneda lemma''' is an abstract result on [[functor]]s of the type ''morphisms into a fixed object''. It is a vast generalisation of [[Cayley's theorem]] from [[group theory]] (viewing a group as a particular kind of category with just one object). It allows the [[Subcategory#Embeddings|embedding]] of any category into a [[category of functors]] (contravariant set-valued functors) defined on that category. It also clarifies how the embedded category, of [[representable functor]]s and their [[natural transformation]]s, relates to the other objects in the larger functor category. It is an important tool that underlies several modern developments in [[algebraic geometry]] and [[representation theory]]. It is named after [[Nobuo Yoneda]].
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| ==Generalities==
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| The Yoneda lemma suggests that instead of studying the ([[locally small]]) category ''C'', one should study the category of all functors of ''C'' into '''Set''' (the [[category of sets]] with [[function (mathematics)|function]]s as [[morphism]]s). '''Set''' is a category we understand well, and a functor of ''C'' into '''Set''' can be seen as a "representation" of ''C'' in terms of known structures. The original category ''C'' is contained in this functor category, but new objects appear in the functor category which were absent and "hidden" in ''C''. Treating these new objects just like the old ones often unifies and simplifies the theory.
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| This approach is akin to (and in fact generalizes) the common method of studying a [[ring (mathematics)|ring]] by investigating the [[module (mathematics)|modules]] over that ring. The ring takes the place of the category ''C'', and the category of modules over the ring is a category of functors defined on ''C''.
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| ==Formal statement==
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| ===General version===
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| Yoneda's lemma concerns functors from a fixed category ''C'' to the [[category of sets]], '''Set'''. If ''C'' is a [[locally small category]] (i.e. the [[hom-set]]s are actual sets and not proper classes), then each object ''A'' of ''C'' gives rise to a natural functor to '''Set''' called a [[hom-functor]]. This functor is denoted:
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| :<math>h^A = \mathrm{Hom}(A,-).</math>
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| The ([[Covariance and contravariance of functors|covariant]]) hom-functor ''h''<sup>''A''</sup> sends ''X'' to the set of [[morphism]]s Hom(''A'',''X'') and sends a morphism ''f'' from ''X'' to ''Y'' to the morphism <math>f \circ -</math> (composition with ''f'' on the left) that sends a morphism ''g'' in Hom(''A'',''X'') to the morphism ''f'' o ''g'' in Hom(''A'',''Y''). That is,
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| : <math>f \longmapsto \mathrm{Hom}(A,f) = [\![ \mathrm{Hom}(A,X) \ni g \mapsto f \circ g \in \mathrm{Hom}(A,Y) ]\!] </math>.
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| Let ''F'' be an arbitrary functor from ''C'' to '''Set'''. Then Yoneda's lemma says that:
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| For each object ''A'' of ''C'', the [[natural transformation]]s from ''h''<sup>''A''</sup> to ''F'' are in one-to-one correspondence with the elements of ''F''(''A''). That is,
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| :<math>\mathrm{Nat}(h^A,F) \cong F(A).</math>
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| Moreover this isomorphism is natural in ''A'' and ''F'' when both sides are regarded as functors from '''Set'''<sup>''C''</sup> x C to '''Set'''.
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| Given a natural transformation Φ from ''h''<sup>''A''</sup> to ''F'', the corresponding element of ''F''(''A'') is <math>u = \Phi_A(\mathrm{id}_A)</math>.{{efn|Recall that <math> \Phi_A : \mathrm{Hom}(A,A) \to F(A) </math> so the last expression is well-defined and sends a morphism from ''A'' to ''A'', to an object in ''F''(''A'').}}
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| There is a contravariant version of Yoneda's lemma which concerns [[contravariant functor]]s from ''C'' to '''Set'''. This version involves the contravariant hom-functor
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| :<math>h_A = \mathrm{Hom}(-, A),</math>
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| which sends ''X'' to the hom-set Hom(''X'',''A''). Given an arbitrary contravariant functor ''G'' from ''C'' to '''Set''', Yoneda's lemma asserts that
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| :<math>\mathrm{Nat}(h_A,G) \cong G(A).</math>
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| ===Naming conventions===
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| The use of "''h''<sup>''A''</sup>" for the covariant hom-functor and "''h''<sub>''A''</sub>" for the contravariant hom-functor is not completely standard. Many texts and articles either use the opposite convention or completely unrelated symbols for these two functors. However, most modern algebraic geometry texts starting with [[Alexander_Grothendieck|Alexander Grothendieck's]] foundational [[Éléments_de_géométrie_algébrique|EGA]] use the convention in this article.{{efn|A notable exception to modern algebraic geometry texts following the conventions of this article is ''Commutative algebra with a view toward algebraic geometry'' / David Eisenbud (1995), which uses "''h''<sub>''A''</sub>" to mean the covariant hom-functor. However, the later book ''The geometry of schemes'' / David Eisenbud, Joe Harris (1998) reverses this and uses "''h''<sub>''A''</sub>" to mean the contravariant hom-functor.}}
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| The mnemonic "falling into something" can be helpful in remembering that "''h''<sub>''A''</sub>" is the contravariant hom-functor. When the letter "''A''" is '''falling''' (i.e. a subscript), ''h''<sub>''A''</sub> assigns to an object ''X'' the morphisms from ''X'' '''into''' ''A''.
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| ===Proof===
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| The proof of Yoneda's lemma is indicated by the following [[commutative diagram]]:
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| [[Image:YonedaLemma-02.png|center|Proof of Yoneda's lemma]]
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| This diagram shows that the natural transformation Φ is completely determined by <math>\Phi_A(\mathrm{id}_A)=u</math> since for each morphism ''f'' : ''A'' → ''X'' one has
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| :<math>\Phi_X(f) = (Ff)u.\,</math>
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| Moreover, any element ''u''∈''F''(''A'') defines a natural transformation in this way. The proof in the contravariant case is completely analogous.
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| In this way, Yoneda's Lemma provides a complete classification of all natural transformations from the functor Hom(A,-) to an arbitrary functor F:C→Set.
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| ===The Yoneda embedding===
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| An important special case of Yoneda's lemma is when the functor ''F'' from ''C'' to '''Set''' is another hom-functor ''h''<sup>''B''</sup>. In this case, the covariant version of Yoneda's lemma states that
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| :<math>\mathrm{Nat}(h^A,h^B) \cong \mathrm{Hom}(B,A).</math>
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| That is, natural transformations between hom-functors are in one-to-one correspondence with morphisms (in the reverse direction) between the associated objects. Given a morphism ''f'' : ''B'' → ''A'' the associated natural transformation is denoted Hom(''f'',–).
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| Mapping each object ''A'' in ''C'' to its associated hom-functor ''h''<sup>''A''</sup> = Hom(''A'',–) and each morphism ''f'' : ''B'' → ''A'' to the corresponding natural transformation Hom(''f'',–) determines a contravariant functor ''h''<sup>–</sup> from ''C'' to '''Set'''<sup>''C''</sup>, the [[functor category]] of all (covariant) functors from ''C'' to '''Set'''. One can interpret ''h''<sup>–</sup> as a [[covariant functor]]:
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| :<math>h^{-}\colon \mathcal C^{\text{op}} \to \mathbf{Set}^\mathcal C.</math>
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| The meaning of Yoneda's lemma in this setting is that the functor ''h''<sup>–</sup> is [[full and faithful functors|fully faithful]], and therefore gives an embedding of ''C''<sup>op</sup> in the category of functors to '''Set'''. The collection of all functors {''h''<sup>A</sup>, A in C} is a subcategory of '''Set'''<sup>''C''</sup>. Therefore, Yoneda embedding implies that the category '''C'''<sup>''op''</sup> is isomorphic to the category {''h''<sup>A</sup>, A in C}.
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| The contravariant version of Yoneda's lemma states that
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| :<math>\mathrm{Nat}(h_A,h_B) \cong \mathrm{Hom}(A,B).</math>
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| Therefore, ''h''<sub>–</sub> gives rise to a covariant functor from ''C'' to the category of contravariant functors to '''Set''':
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| :<math>h_{-}\colon \mathcal C \to \mathbf{Set}^{\mathcal C^{\mathrm{op}}}.</math>
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| Yoneda's lemma then states that any locally small category ''C'' can be embedded in the category of contravariant functors from ''C'' to '''Set''' via ''h''<sub>–</sub>. This is called the ''Yoneda embedding''.
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| ==Preadditive categories, rings and modules==
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| A ''[[preadditive category]]'' is a category where the morphism sets form [[abelian group]]s and the composition of morphisms is [[bilinear operator|bilinear]]; examples are categories of abelian groups or modules. In a preadditive category, there is both a "multiplication" and an "addition" of morphisms, which is why preadditive categories are viewed as generalizations of [[ring (mathematics)|ring]]s. Rings are preadditive categories with one object.
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| The Yoneda lemma remains true for preadditive categories if we choose as our extension the category of ''[[additive functor|additive]]'' contravariant functors from the original category into the category of abelian groups; these are functors which are compatible with the addition of morphisms and should be thought of as forming a ''[[module category]]'' over the original category. The Yoneda lemma then yields the natural procedure to enlarge a preadditive category so that the enlarged version remains preadditive — in fact, the enlarged version is an [[abelian category]], a much more powerful condition. In the case of a ring ''R'', the extended category is the category of all right [[module (mathematics)|modules]] over ''R'', and the statement of the Yoneda lemma reduces to the well-known isomorphism
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| :''M'' ≅ Hom<sub>''R''</sub>(''R'',''M'') for all right modules ''M'' over ''R''.
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| ==History==
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| The Yoneda lemma was introduced but not proved in a 1954 paper by Nobuo Yoneda.<ref>{{cite journal|last=Nobuo|first=Yoneda|title=On the homology theory of modules|journal=J. Fac. Sci. Univ. Tokyo. Sect. I|year=1954|volume=7|pages=193-227|url=http://www.researchgate.net/publication/247041022_ON_THE_HOMOLOGY_THEORY_OF_MODULES|accessdate=21 December 2013}} {{Subscription required}}</ref> Yoshiki Kinoshita stated in 1996 that the term "Yoneda lemma" was coined by [[Saunders Mac Lane]] following an interview he had with Yoneda.<ref>{{cite web|title=Prof. Nobuo Yoneda passed away|last=Kinoshita|first=Yoshiki|url=http://www.mta.ca/~cat-dist/catlist/1999/yoneda|accessdate=21 December 2013|date=23 April 1996}}</ref>
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| ==See also==
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| * [[Representation theorem]]
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| ==Notes==
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| {{Notelist}}
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| ==References==
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| {{Reflist}}
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| {{Refbegin}}
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| * {{citation | last=Freyd | first=Peter | author-link=Peter J. Freyd | title=Abelian categories | publisher=Harper and Row | year=1964 | url=http://www.tac.mta.ca/tac/reprints/articles/3/tr3abs.html | edition=2003 reprint | zbl=0121.02103 | series=Harper's Series in Modern Mathematics }}.
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| * {{citation | last=Mac Lane | first=Saunders | author-link=Saunders Mac Lane | title=[[Categories for the Working Mathematician]] | edition=2nd | series=[[Graduate Texts in Mathematics]] | volume=5 | location=New York, NY | publisher=[[Springer-Verlag]] | year=1998 | isbn=0-387-98403-8 | zbl=0906.18001 }}
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| {{Refend}}
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| * {{nlab|id=Yoneda+lemma|title=Yoneda lemma}}
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| [[Category:Representable functors]]
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| [[Category:Lemmas]]
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| [[Category:Articles containing proofs]]
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Hi there! :) My name is Lin, I'm a student studying Anthropology and Sociology from Syracuse, United States.
Feel free to visit my website ... Fifa 15 Coin generator