Reflective subcategory
# Reflective subcategory

In mathematics, a full subcategory A of a category B is said to be reflective in B when the inclusion functor from A to B has a left $adjoint.^{:91}$ This adjoint is sometimes called a reflector, or localization. Dually, A is said to be coreflective in B when the inclusion functor has a right adjoint.

Informally, a reflector acts as a kind of completion operation. It adds in any "missing" pieces of the structure in such a way that reflecting it again has no further effect.

## Definition

A full subcategory A of a category B is said to be reflective in B if for each B-object B there exists an A-object $A_{B}$ and a B-morphism $r_{B}\colon B\to A_{B}$ such that for each B-morphism $f\colon B\to A$ to an A-object $A$ there exists a unique A-morphism ${\overline {f}}\colon A_{B}\to A$ with ${\overline {f}}\circ r_{B}=f$.

The pair $(A_{B},r_{B})$ is called the A-reflection of B. The morphism $r_{B}$ is called the A-reflection arrow. (Although often, for the sake of brevity, we speak about $A_{B}$ only as being the A-reflection of B).

This is equivalent to saying that the embedding functor $E\colon \mathbf {A} \hookrightarrow \mathbf {B}$ is a right adjoint. The left adjoint functor $R\colon \mathbf {B} \to \mathbf {A}$ is called the reflector. The map $r_{B}$ is the unit of this adjunction.

The reflector assigns to $B$ the A-object $A_{B}$ and $Rf$ for a B-morphism $f$ is determined by the commuting diagram

If all A-reflection arrows are (extremal) epimorphisms, then the subcategory A is said to be (extremal) epireflective. Similarly, it is bireflective if all reflection arrows are bimorphisms.

All these notions are special case of the common generalization—$E$-reflective subcategory, where $E$ is a class of morphisms.

The $E$-reflective hull of a class A of objects is defined as the smallest $E$-reflective subcategory containing A. Thus we can speak about reflective hull, epireflective hull, extremal epireflective hull, etc.

An anti-reflective subcategory is a full subcategory A such that the only objects of B that have an A-reflection arrow are those that are already in A.

Dual notions to the above-mentioned notions are coreflection, coreflection arrow, (mono)coreflective subcategory, coreflective hull, anti-coreflective subcategory.

## Examples

### Algebra

* The category of abelian groups Ab is a reflective subcategory of the category of groups, Grp. The reflector is the functor that sends each group to its abelianization. In its turn, the category of groups is a reflective subcategory of the category of inverse semigroups.
* Similarly, the category of commutative associative algebras is a reflective subcategory of all associative algebras, where the reflector is quotienting out by the commutator ideal. This is used in the construction of the symmetric algebra from the tensor algebra.
* Dually, the category of anti-commutative associative algebras is a reflective subcategory of all associative algebras, where the reflector is quotienting out by the anti-commutator ideal. This is used in the construction of the exterior algebra from the tensor algebra.
* The category of fields is a reflective subcategory of the category of integral domains (with injective ring homomorphisms as morphisms). The reflector is the functor that sends each integral domain to its field of fractions.
* The category of abelian torsion groups is a coreflective subcategory of the category of abelian groups. The coreflector is the functor sending each group to its torsion subgroup.
* The categories of elementary abelian groups, abelian p-groups, and p-groups are all reflective subcategories of the category of groups, and the kernels of the reflection maps are important objects of study; see focal subgroup theorem.
* The category of groups is a coreflective subcategory of the category of monoids: the right adjoint maps a monoid to its group of units.

### Topology

* The category of Kolmogorov spaces $(T_0$ spaces) is a reflective subcategory of Top, the category of topological spaces, and the Kolmogorov quotient is the reflector.
* The category of completely regular spaces CReg is a reflective subcategory of Top. By taking Kolmogorov quotients, one sees that the subcategory of Tychonoff spaces is also reflective.
* The category of all compact Hausdorff spaces is a reflective subcategory of the category of all Tychonoff spaces (and of the category of all topological $spaces^{:140}).$ The reflector is given by the Stone–Čech compactification.
* The category of all complete metric spaces with uniformly continuous mappings is a reflective subcategory of the category of metric spaces. The reflector is the completion of a metric space on objects, and the extension by density on $arrows.^{:90}$
* The category of sheaves is a reflective subcategory of presheaves on a topological space. The reflector is sheafification, which assigns to a presheaf the sheaf of sections of the bundle of its germs.

### Functional analysis

* The category of Banach spaces is a reflective subcategory of the category of normed spaces and bounded linear operators. The reflector is the norm completion functor.

### Category theory

* For any Grothendieck site (C, J), the topos of sheaves on (C, J) is a reflective subcategory of the topos of presheaves on C, with the special further property that the reflector functor is left exact. The reflector is the sheafification functor a : Presh(C) → Sh(C, J), and the adjoint pair (a, i) is an important example of a geometric morphism in topos theory.

## Properties

* The components of the counit are $isomorphisms.^{:140}$
* If D is a reflective subcategory of C, then the inclusion functor D → C creates all limits that are present in $C.^{:141}$
* A reflective subcategory has all colimits that are present in the ambient $category.^{:141}$
* The monad induced by the reflector/localization adjunction is $idempotent.^{:158}$
