Representable functor (original) (raw)

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In mathematics, particularly category theory, a representable functor is a certain functor from an arbitrary category into the category of sets. Such functors give representations of an abstract category in terms of known structures (i.e. sets and functions) allowing one to utilize, as much as possible, knowledge about the category of sets in other settings.

From another point of view, representable functors for a category C are the functors given with C. Their theory is a vast generalisation of upper sets in posets, and Yoneda's representability theorem generalizes Cayley's theorem in group theory.

Let C be a locally small category and let Set be the category of sets. For each object A of C let Hom(A,–) be the hom functor that maps object X to the set Hom(A,X).

A functor F : C → Set is said to be representable if it is naturally isomorphic to Hom(A,–) for some object A of C. A representation of F is a pair (A, Φ) where

Φ : Hom(A,–) → F

is a natural isomorphism.

A contravariant functor G from C to Set is the same thing as a functor G : Cop → Set and is commonly called a presheaf. A presheaf is representable when it is naturally isomorphic to the contravariant hom-functor Hom(–,A) for some object A of C.

According to Yoneda's lemma, natural transformations from Hom(A,–) to F are in one-to-one correspondence with the elements of F(A). Given a natural transformation Φ : Hom(A,–) → F the corresponding element u ∈ F(A) is given by

u = Φ A ( i d A ) . {\displaystyle u=\Phi _{A}(\mathrm {id} _{A}).\,} {\displaystyle u=\Phi _{A}(\mathrm {id} _{A}).\,}

Conversely, given any element u ∈ F(A) we may define a natural transformation Φ : Hom(A,–) → F via

Φ X ( f ) = ( F f ) ( u ) {\displaystyle \Phi _{X}(f)=(Ff)(u)\,} {\displaystyle \Phi _{X}(f)=(Ff)(u)\,}

where f is an element of Hom(A,X). In order to get a representation of F we want to know when the natural transformation induced by u is an isomorphism. This leads to the following definition:

A universal element of a functor F : C → Set is a pair (A,u) consisting of an object A of C and an element u ∈ F(A) such that for every pair (X,v) consisting of an object X of C and an element v ∈ F(X) there exists a unique morphism f : A → X such that (Ff)(u) = v.

A universal element may be viewed as a universal morphism from the one-point set {•} to the functor F or as an initial object in the category of elements of F.

The natural transformation induced by an element u ∈ F(A) is an isomorphism if and only if (A,u) is a universal element of F. We therefore conclude that representations of F are in one-to-one correspondence with universal elements of F. For this reason, it is common to refer to universal elements (A,u) as representations.

Analogy: Representable functionals

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Consider a linear functional on a complex Hilbert space H, i.e. a linear function F : H → C {\displaystyle F:H\to \mathbb {C} } {\displaystyle F:H\to \mathbb {C} }. The Riesz representation theorem states that if F is continuous, then there exists a unique element a ∈ H {\displaystyle a\in H} {\displaystyle a\in H} which represents F in the sense that F is equal to the inner product functional ⟨ a , − ⟩ {\displaystyle \langle a,-\rangle } {\displaystyle \langle a,-\rangle }, that is F ( v ) = ⟨ a , v ⟩ {\displaystyle F(v)=\langle a,v\rangle } {\displaystyle F(v)=\langle a,v\rangle } for v ∈ H {\displaystyle v\in H} {\displaystyle v\in H}.

For example, the continuous linear functionals on the square-integrable function space H = L 2 ( R ) {\displaystyle H=L^{2}(\mathbb {R} )} {\displaystyle H=L^{2}(\mathbb {R} )} are all representable in the form F ( v ) = ⟨ a , v ⟩ = ∫ R a ( x ) v ( x ) d x {\displaystyle \textstyle F(v)=\langle a,v\rangle =\int _{\mathbb {R} }a(x)v(x)\,dx} {\displaystyle \textstyle F(v)=\langle a,v\rangle =\int _{\mathbb {R} }a(x)v(x)\,dx} for a unique function a ( x ) ∈ H {\displaystyle a(x)\in H} {\displaystyle a(x)\in H}. The theory of distributions considers more general continuous functionals on the space of test functions C = C c ∞ ( R ) {\displaystyle C=C_{c}^{\infty }(\mathbb {R} )} {\displaystyle C=C_{c}^{\infty }(\mathbb {R} )}. Such a distribution functional is not necessarily representable by a function, but it may be considered intuitively as a generalized function. For instance, the Dirac delta function is the distribution defined by F ( v ) = v ( 0 ) {\displaystyle F(v)=v(0)} {\displaystyle F(v)=v(0)} for each test function v ( x ) ∈ C {\displaystyle v(x)\in C} {\displaystyle v(x)\in C}, and may be thought of as "represented" by an infinitely tall and thin bump function near x = 0 {\displaystyle x=0} {\displaystyle x=0}.

Thus, a function a ( x ) {\displaystyle a(x)} {\displaystyle a(x)} may be determined not by its values, but by its effect on other functions via the inner product. Analogously, an object A in a category may be characterized not by its internal features, but by its functor of points, i.e. its relation to other objects via morphisms. Just as non-representable functionals are described by distributions, non-representable functors may be described by more complicated structures such as stacks.

Representations of functors are unique up to a unique isomorphism. That is, if (_A_1,Φ1) and (_A_2,Φ2) represent the same functor, then there exists a unique isomorphism φ : _A_1 → _A_2 such that

Φ 1 − 1 ∘ Φ 2 = H o m ( φ , − ) {\displaystyle \Phi _{1}^{-1}\circ \Phi _{2}=\mathrm {Hom} (\varphi ,-)} {\displaystyle \Phi _{1}^{-1}\circ \Phi _{2}=\mathrm {Hom} (\varphi ,-)}

as natural isomorphisms from Hom(_A_2,–) to Hom(_A_1,–). This fact follows easily from Yoneda's lemma.

Stated in terms of universal elements: if (_A_1,_u_1) and (_A_2,_u_2) represent the same functor, then there exists a unique isomorphism φ : _A_1 → _A_2 such that

( F φ ) u 1 = u 2 . {\displaystyle (F\varphi )u_{1}=u_{2}.} {\displaystyle (F\varphi )u_{1}=u_{2}.}

Preservation of limits

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Representable functors are naturally isomorphic to Hom functors and therefore share their properties. In particular, (covariant) representable functors preserve all limits. It follows that any functor which fails to preserve some limit is not representable.

Contravariant representable functors take colimits to limits.

Any functor K : C → Set with a left adjoint F : Set → C is represented by (FX, η_X_(•)) where X = {•} is a singleton set and η is the unit of the adjunction.

Conversely, if K is represented by a pair (A, u) and all small copowers of A exist in C then K has a left adjoint F which sends each set I to the _I_th copower of A.

Therefore, if C is a category with all small copowers, a functor K : C → Set is representable if and only if it has a left adjoint.

Relation to universal morphisms and adjoints

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The categorical notions of universal morphisms and adjoint functors can both be expressed using representable functors.

Let G : D → C be a functor and let X be an object of C. Then (A,φ) is a universal morphism from X to G if and only if (A,φ) is a representation of the functor Hom_C_(X,G_–) from D to Set. It follows that G has a left-adjoint F if and only if Hom_C(X,G_–) is representable for all X in C. The natural isomorphism Φ_X : Hom_D_(FX,–) → Hom_C_(X,_G_–) yields the adjointness; that is

Φ X , Y : H o m D ( F X , Y ) → H o m C ( X , G Y ) {\displaystyle \Phi _{X,Y}\colon \mathrm {Hom} _{\mathcal {D}}(FX,Y)\to \mathrm {Hom} _{\mathcal {C}}(X,GY)} {\displaystyle \Phi _{X,Y}\colon \mathrm {Hom} _{\mathcal {D}}(FX,Y)\to \mathrm {Hom} _{\mathcal {C}}(X,GY)}

is a bijection for all X and Y.

The dual statements are also true. Let F : C → D be a functor and let Y be an object of D. Then (A,φ) is a universal morphism from F to Y if and only if (A,φ) is a representation of the functor Hom_D_(F_–,Y) from C to Set. It follows that F has a right-adjoint G if and only if Hom_D(_F_–,Y) is representable for all Y in D.[2]

  1. ^ Hungerford, Thomas. Algebra. Springer-Verlag. p. 470. ISBN 3-540-90518-9.
  2. ^ Nourani, Cyrus. A Functorial Model Theory: Newer Applications to Algebraic Topology, Descriptive Sets, and Computing Categories Topos. CRC Press. p. 28. ISBN 1482231506.