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. This adjoint is sometimes called a reflector. 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 $\overline f\circ r_B=f$.

File:Refl1.png

The pair $(A_B,r_B)$ is called the A-reflection of B. The morphism $r_B$ is called A-reflection arrow. (Although often, for the sake of brevity, we speak about $A_B$ only as about the A-reflection of B).

This is equivalent to saying that the embedding functor $E\colon \mathbf{A} \hookrightarrow \mathbf{B}$ is adjoint. The coadjoint 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

File:Reflsq1.png

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 which 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 which 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.

Topology

• Kolmogorov spaces (T₀ spaces) are 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. 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.

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.