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Adjoint functors can be considered from several different points of view. This article starts with a number of introductory sections considering some of these. MOTIVATION Ubiquity of adjoint functors The idea of an adjoint functor was formulated by Daniel Kan in 1958 . Like many of the concepts in category theory, it was suggested by the needs of Homological Algebra , which was at the time devoted to computations. Those faced with giving tidy, systematic presentations of the subject would have noticed relations such as :Hom(''F''(''X''), ''Y'') = Hom(''X'', ''G''(''Y'')) in the category of Abelian Group s, where ''F'' was the functor –⊗''A'' (i.e. take the Tensor Product with ''A''), and ''G'' was the functor Hom(''A'',–). The use of the ''equals'' sign is an . The terminology comes from the Hilbert Space idea of Adjoint Operator s ''T'', ''U'' with <''Tx'',''y''> = <''x'',''Uy''>, which is formally similar to the above Hom relation. We say that ''F'' is ''left adjoint'' to ''G'', and ''G'' is ''right adjoint'' to ''F''. Since ''G'' may have itself a right adjoint, quite different from ''F'' (see below for an example). The analogy to adjoint maps of Hilbert spaces can be made precise in certain contexts {Link without Title} . If one starts looking for these adjoint pairs of functors, they turn out to be very common in Abstract Algebra , and elsewhere as well. The example section below provides evidence of this; furthermore, Universal Construction s, which may be more familiar to some, give rise to numerous adjoint pairs of functors. In accordance with the thinking of Saunders Mac Lane , any idea such as adjoint functors that occurs widely enough in mathematics should be studied for its own sake. Deep problems formulated with adjoint functors By itself, the ''generality'' of the adjoint functor concept isn't a recommendation to most mathematicians. Concepts are judged according to their use in solving problems, at least as much as for their use in building theories. The tension between these two potential motivations for developing a mathematical concept was especially great during the 1950s when category theory was initially developed. Enter Alexander Grothendieck , who used category theory to take compass bearings in foundational, axiomatic work — in Functional Analysis , Homological Algebra and finally Algebraic Geometry . It is probably wrong to say that he promoted the adjoint functor concept in isolation: but recognition of the role of adjunction was inherent in Grothendieck's approach. For example, one of his major achievements was the formulation of Serre Duality in relative form — one could say loosely, in a continuous family of algebraic varieties. The entire proof turned on the existence of a right adjoint to a certain functor. This is something undeniably abstract, and non-constructive, but also powerful in its own way. Adjoint functors as solving optimization problems One good way to motivate adjoint functors is to explain what problem they solve, and how they solve it. That can only be done, in some sense, by what mathematicians call ' and Glossary Of Ring Theory ). The ''best'' way is to add an element '1' to the ring, add nothing extra you don't need (you will need to have ''r''+1 for ''r'' in the ring, clearly), and add no relations in the new ring that aren't forced by axioms. This is rather vague, though suggestive. There are several ways to make precise this concept of ''best structure''. Adjoint functors are one method; the notion of Universal Properties provides another, essentially equivalent but arguably more concrete approach.
The adjoint functor method for defining a multiplicative identity for rings is to look at two categories, ''C''0 and ''C''1, of rings, respectively without and with assumption of multiplicative identity. There is a functor from ''C''1 to ''C''0 that forgets about the 1. We are seeking a left adjoint to it. This is a clear, if dry, formulation. One way to see what is achieved by using either formulation is to try a direct method. (This is favoured, for example, by John Conway .) One simply adds to ''R'' a new element 1, and calculates on the basis that any equation resulting is valid ''if and only if it holds for all rings'' that we can create from ''R'' and 1. This is the Impredicative method: meaning that the ring we are trying to construct is one of the rings quantified over in 'all rings'. This overt use of impredicativity is ''honest'', in a way that category theory has no intention of being. The ''answer'' regarding the way to get a ( Unital ) ring from one that is not unital is simple enough (see examples below); this section has been a discussion of how to formulate the question. The major argument in favour of adjoint functors is probably this: if one goes through the universal property or impredicative reasoning often enough, it seems like repeating the same kind of steps. The case of partial orders Every of course is a leading one. Any Galois connection gives rise to Closure Operator s and to inverse order-preserving bijections between the corresponding closed elements. As is the case for Galois groups, the real interest lies often in refining a correspondence to a Duality (i.e. ''antitone'' order isomorphism). A treatment of Galois theory along these lines by Kaplansky was influential in the recognition of the general structure here. The partial order case collapses the adjunction definitions quite noticeably, but can provide several themes:
Together these observations provide explanatory value all over mathematics. FORMAL DEFINITIONS A pair of adjoint functors between two categories ''C'' and ''D'' consists of two :φ : Mor''D''(''F''–, –) → Mor''C''(–, ''G''–) consisting of Bijection s: :φ''X'',''Y'' : Mor''D''(''F''(''X''), ''Y'') → Mor''C''(''X'', ''G''(''Y'')) for all objects ''X'' in ''C'' and ''Y'' in ''D''. Here the naturality of φ means that for all : The horizontal arrows in this diagram are those induced by ''f'' and ''g''. We then say that ''F'' is a left-adjoint of ''G'' and ''G'' is a '''right-adjoint''' of ''F'', and often write ''F''⊣''G''. At this point, one might wonder why we have called φ a ''natural isomorphism''. In fact, we can express the definition of the naturality of φ given above, so that φ truly is a natural isomorphism (under the usual definition of natural isomorphism). The way to do this is to interpret hom-sets as actual functors between suitable categories. To be precise, for any category ''C'', there is a hom-functor Mor(–, –) : ''C''Op × ''C'' → Set. If we interpret the "''F''" in Mor''D''(''F''–, –) as really representing the "opposite functor" ''F''Op × '''id'''''D'' : ''C''Op × ''D'' → ''D''Op × ''D'', defined in the expected way, and if we interpret the "G" in Mor''C''(–, ''G''–) as representing the functor '''id'''''C''Op × ''G'' : ''C''Op × ''D'' → ''C''Op × ''C'', defined in the expected way, then each of Mor''D''(''F''–, –) and Mor''C''(–, ''G''–) is in fact a functor from ''C''Op × ''D'' → Set, and the "naturality" requirement is just equivalent to requiring that these two functors be naturally isomorphic. Unit and Co-unit Every adjoint pair of functors defines a unit η, a Natural Transformation from the functor 1''C'' to ''GF'' consisting of morphisms :η''X'' : ''X'' → ''GF''(''X'') for every ''X'' in ''C''. η''X'' is defined as φ''X,F''(''X'') (id''F''(''X'')). Analogously, one may define a co-unit ε, a natural transformation from ''FG'' to 1''D'' consisting of morphisms :ε''Y'' : ''FG''(''Y'') → ''Y''. for every ''Y'' in ''D''. ε''Y'' is defined as φ''G''(''Y''),''Y''−1(id''G''(''Y'')). If the unit and counit are actually isomorphisms, then the adjunction provides an Equivalence Of Categories . For this reason, an adjunction may be considered an even weaker form of equivalence of categories. EXAMPLES → Grp''' is the functor assigning to each set ''X'' the Free Group over ''X'', and if ''G'' : '''Grp''' → '''Set''' is the forgetful functor assigning to each group its underlying set, then the universal property of the free group shows that ''F'' is left adjoint to ''G''. The unit of this adjoint pair is the embedding of a set ''X'' into the free group over ''X''. In general, free constructions in mathematics tend to be left adjoints of forgetful functors. Free Ring s, Free Abelian Group s, and Free Module s follow this general pattern. which assigns to every group ''X'' the pair (''X'', ''X'') in the product category Grp2''', and Π : '''Grp2''' → '''Grp''' the functor which assigns to each pair (''Y''1, ''Y''2) the product group ''Y''1×''Y''2. The universal property of the product group shows that ''G'' is right-adjoint to ''F''. The co-unit gives the natural projections from the product to the factors. The Cartesian Product of Sets , the product of rings, the Product Of Topological Spaces etc. follow the same pattern; it can also be extended in a straightforward manner to more than just two factors. In fact, any Limit or colimit is adjoint to a diagonal functor. assigns to every pair (''X''1, ''X''2) of abelian groups their Direct Sum and if ''G'' : Ab''' → '''Ab2''' is the functor which assigns to every abelian group ''Y'' the pair (''Y'', ''Y''), then ''F'' is left adjoint to ''G'', again a consequence of the universal property of direct sums. The unit of the adjoint pair provides the natural embeddings from the factors into the direct sum. Analogous examples are given by the Direct Sum of Vector Spaces and Modules , by the Free Product of groups and by the disjoint union of sets. Kernels. Consider the category ''D'' of homomorphisms of abelian groups. If ''f''1 : ''A''1 → ''B''1 and ''f''2 : ''A''2 → ''B''2 are two objects of ''D'', then a morphism from ''f''1 to ''f''2 is a pair (''g''''A'', ''g''''B'') of morphisms such that ''gBf''1 = ''f''2''gA''. Let ''G'' : ''D'' → '''Ab''' be the functor which assigns to each homomorphism its Kernel and let ''F'' : '''Ab''' → ''D'' be the morphism which maps the group ''A'' to the homomorphism ''A'' → 0. Then ''G'' is right adjoint to ''F'', which expresses the universal property of kernels, and the co-unit of this adjunction yields the natural embedding of a homomorphism's kernel into the homomorphism's domain. A suitable variation of this example also shows that the kernel functors for vector spaces and for modules are right adjoints. Analogously, one can show that the cokernel functors for abelian groups, vector spaces and modules are left adjoints. Making a ring Unital This example was discussed in section 1.3 above. Given a non-unital ring ''R'', a multiplicative identity element can be added by taking ''R''x'''Z''' and defining a '''Z'''-bilinear product with (r,0)(0,1) = (0,1)(r,0) = (r,0), (r,0)(s,0) = (rs,0), (0,1)(0,1) = (0,1). This constructs a left adjoint to the functor taking a ring to the underlying non-unital ring. Ring extensions. Suppose ''R'' and ''S'' are rings, and ρ : ''R'' → ''S'' is a Ring Homomorphism . Then ''S'' can be seen as a (left) ''R''-module, and the Tensor Product with ''S'' yields a functor ''F'' : ''R''-'''Mod''' → ''S''-'''Mod'''. Then ''F'' is left adjoint to the forgetful functor ''G'' : ''S''-'''Mod''' → ''R''-'''Mod'''. Tensor products. If ''R'' is a ring and ''M'' is a right ''R'' module, then the tensor product with ''M'' yields a functor ''F'' : ''R''-'''Mod''' → '''Ab'''. The functor ''G'' : '''Ab''' → ''R''-'''Mod''', defined by ''G''(''A'') = Hom'''Z'''(''A'', ''M'') for every abelian group ''A'', is a right adjoint to ''F''. From monoids and groups to rings The Monoid Ring construction gives a functor from Monoid s to rings. This functor is left adjoint to the functor that associates to a given ring its underlying multiplicative monoid. Similarly, the Group Ring construction yields a functor from Group s to rings, left adjoint to the functor that assigns to a given ring its Group Of Units . One can also start with a Field ''K'' and consider the category of ''K''- Algebras instead of the category of rings, to get the monoid and group rings over ''K''. Direct and inverse images of sheaves Every s induces a functor ''f'' ∗ from the category of Sheaves (of sets, or abelian groups, or rings...) on ''X'' to the corresponding category of sheaves on ''Y'', the '' Direct Image Functor ''. It also induces a functor ''f'' ∗ from the category of sheaves on ''Y'' to the category of sheaves on ''X'', the '' Inverse Image Functor ''. ''f'' ∗ is left adjoint to ''f'' ∗. Here a more subtle point is that the left adjoint for Coherent Sheaves will differ from that for sheaves (of sets). The Grothendieck construction. In K-theory , the point of departure is to observe that the category of Vector Bundle s on a Topological Space has a commutative monoid structure under Direct Sum . To make an Abelian Group out of this monoid, one can follow the method of making a Presentation Of A Group , adding formally an additive inverse for each bundle (or equivalence class). Alternatively one can observe that the functor that for each group takes the underlying monoid (ignoring inverses) has a left adjoint. This is a once-for-all construction, in line with the third section discussion above. That is, one can imitate the construction of Negative Number s; but there is the other option of an Existence Theorem . For the case of finitary algebraic structures, the existence by itself can be referred to Universal Algebra , or Model Theory ; naturally there is also a proof adapted to category theory, too. Frobenius reciprocity in the . This example foreshadowed the general theory by about half a century. Stone-Čech compactification. Let ''D'' be the category of . The unit of this adjoint pair yields a Continuous map from every topological space ''X'' into its Stone-Čech compactification. This map is an Embedding (i.e. Injective , continuous and open) if and only if ''X'' is a Tychonoff Space . Soberification. The article on Stone Duality describes an adjunction between the category of topological spaces and the category of Sober Space s that is known as soberification. Notably, the article also contains a detailed description of another adjunction that prepares the way for the famous Duality of sober spaces and spatial locales, exploited in Pointless Topology . A functor with a left and a right adjoint. Let ''G'' be the functor from Topological Space s to Set s that associates to every topological space its underlying set (forgetting the topology, that is). ''G'' has a left adjoint ''F'', creating the Discrete Space on a set ''Y'', and a right adjoint ''H'' creating the Trivial Topology on ''Y''. PROPERTIES Uniqueness of adjoints If the functor ''F'' : ''C'' → ''D'' had two right-adjoints ''G''1 and ''G''2, then ''G''1 and ''G''2 are Naturally Isomorphic . The same is true for left-adjoints. Relation to universal constructions All pairs of adjoint functors arise from Universal Constructions . Let ''F'' and ''G'' be a pair of adjoint functors with unit η and co-unit ε. Then we have a universal morphism for each object in ''C'' and ''D'':
Conversely, given any two functors ''F'' and ''G'' and Natural Transformation s η : 1''C'' → ''GF'' and ε : ''FG'' → 1''D'' which are universal in the above sense, then ''F'' and ''G'' form an adjoint pair. (Actually it is sufficient to specify only one of η or ε). The isomorphism φ is then determined by the equations f g Universal constructions are more general than adjoint functor pairs: as mentioned earlier, a universal construction is like an optimization problem; it gives rise to an adjoint pair if and only if this problem has a solution for every object of ''C'' (equivalently, every object of ''D''). Characterization via unit and co-unit There exists yet another characterization of adjoint functors via the unit :η : 1''C'' → ''GF'' and co-unit :ε : ''FG'' → 1''D''. These natural transformations have the following properties: the natural transformation :(ε''F'')O(''F''η) : ''F'' → ''FGF'' → ''F'' is equal to 1''F'', and the composition :(''G''ε)o(η''G'') : ''G'' → ''GFG'' → ''G'' is equal to 1''G''. These are called the ''zig-zag equations'' because of the appearance of the corresponding String Diagram s. Conversely, given two natural transformations η and ε with these properties, then the functors ''F'' and ''G'' form an adjoint pair. Adjoints preserve certain limits The most important property of adjoints is their continuity: every functor that has a left adjoint (and therefore ''is'' a right adjoint) is ''continuous'' (i.e. commutes with Limits in the category theoretical sense); every functor that has a right adjoint (and therefore ''is'' a left adjoint) is ''cocontinuous'' (i.e. commutes with Colimits ). Since many common constructions in mathematics are limits or colimits, this provides a wealth of information. For example:
Additivity If the functor ''F'' : ''C'' → ''D'' is left adjoint to ''G'' : ''D'' → ''C'' and both ''C'' and ''D'' are Additive Categories , then both ''F'' and ''G'' are Additive Functor s. Composition If the functor ''F''1 : ''C'' → ''D'' has ''G''1 : ''D'' → ''C'' as right adjoint and the functor ''F''2 : ''D'' → ''E'' has ''G''2 : ''E'' → ''D'' as right adjoint, then the composition ''F''2o''F''1 : ''C'' → ''E'' has ''G''1o''G''2 : ''E'' → ''C'' as right adjoint. Adjoint pairs extend equivalences Every adjoint pair extends an of ''C'' consisting of those objects ''X'' of ''C'' for which η''X'' is an isomorphism, and define ''D''1 as the full subcategory of ''D'' consisting of those objects ''Y'' of ''D'' for which ε''Y'' is an isomorphism. Then ''F'' and ''G'' can be restricted to ''C''1 and ''D''1 and yield inverse equivalences of these subcategories. In a sense, then, adjoints are "generalized" inverses. Note however that a right inverse of ''F'' (i.e. a functor ''G'' such that ''FG'' is naturally isomorphic to 1''D'') need not be a right (or left) adjoint of ''F''. Adjoints generalize ''two-sided'' inverses. General existence theorem Not every functor ''G'' : ''D'' → ''C'' admits a left adjoint. If ''D'' is and a certain smallness condition is satisfied: for every object ''X'' of ''C'' there exists a family of morphisms f (where, to make the point explicitly, the indices ''i'' come from a '' Set '' ''I'', not a '' Proper Class ''—this is the whole idea), such that every morphism h can be written as h for some ''i'' in ''I'' and some morphism t An analogous statement characterizes those functors with a right adjoint. |
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