Contents

Definition

A full subcategory $i:C\hookrightarrow D$ is reflective if the inclusion functor $i$ has a left adjoint $T$:

The left adjoint is sometimes called the reflector, and a functor which is a reflector (or has a fully faithful right adjoint, which is the same up to equivalence) is called a reflection. Of course, there are dual notions of coreflective, coreflector, and coreflection.

The components of the unit

of this adjunction “reflect” each object $d\in D$ into its image $Td$ in the reflective subcategory

This reflection is sometimes called a localization, although sometimes this term is reserved for the case when the functor $T$ is left exact.

If the reflector $T$ is faithful, the reflection is called a completion.

Characterizations

This is due to Gabriel-Zisman.

This is a well-known set of equivalences concerning idempotent monads. The essential point is that a reflective subcategory $i:B\to A$ is monadic, i.e., realizes $B$ as the category of algebras for the monad $ir$ on $A$, where $r:A\to B$ is the reflector.

See also the related discussion at reflective sub-(infinity,1)-category.

Special cases

Exact reflective subcategories

If the reflector (which as a left adjoint always preserves all colimits) in addition preserves finite limits, then the embedding is called exact . If the categories are toposes then such embeddings are called geometric embeddings.

In particular, every sheaf topos is an exact reflective subcategory of a category of presheaves

The reflector in that case is the sheafification functor.

This appears for instance as (Johnstone, A4.3.1).

So in particular if $C$ is an exact reflective subcategory of a cartesian closed category $D$, then $C$ is an exponential ideal of $D$.

See Day's reflection theorem for a more general statement and proof.

Complete reflective subcategories

When the unit of the reflector is a monomorphism, a reflective category is often thought of as a full subcategory of complete objects in some sense; the reflector takes each object in the ambient category to its completion. Such reflective subcategories are sometimes called mono-reflective. One similarly has epi-reflective (when the unit is an epimorphism) and bi-reflective (when the unit is a bimorphism).

In the last case, note that if the unit is an isomorphism, then the inclusion functor is an equivalence of categories, so nontrivial bireflective subcategories can occur only in non-balanced categories. Also note that ‘bireflective’ does not mean reflective and coreflective. One sees this term often in discussions of concrete categories (such as topological categories) where really something stronger holds: that the reflector lies over the identity functor on Set. In this case, one can say that we have a subcategory that is reflective over $\mathrm{Set}$.

Properties

A reflective subcategory is always closed under limits which exist in the ambient category (because the full inclusion is monadic, as noted above), and inherits colimits from the larger category by application of the reflector.

A morphism in a reflective subcategory is monic iff it is monic in the ambient category. A reflective subcategory of a well-powered category is well-powered.

Reflective subcategories of locally presentable categories

Both the weak and strong versions of Vopěnka's principle are equivalent to fairly simple statements concerning reflective subcategories of locally presentable categories:

This is AdamekRosicky, theorem 6.28

(Remark after corollary 6.24 in Adamek-Rosicky book).

Reflective subcategories of cartesian closed categories

In showing that a given category is cartesian closed, the following theorem is often useful (cf. A4.3.1 in the Elephant):

Reflective and coreflective subcategories

This is shown in (BashirVelebil).

Examples

• Complete metric spaces are mono-reflective in metric spaces; the reflector is called completion.

• The unital rings form a mono-reflective subcategory of possibly nonunital rings; the reflector formally adjoins an identity element.

• The category of sheaves on a site $S$ is a reflective subcategory of the category of presheaves on $S$; the reflector is called sheafification. In fact, categories of sheaves are precisely those reflective subcategories of presheaf categories for which the reflector is left exact. This makes the inclusion functor precisely a geometric morphism of topoi.

• A category of concrete presheaves inside a category of presheaves on a concrete site is a reflective subcategory.

Property vs structure

Whenever $C$ is a full subcategory of $D$, we can say that objects of $C$ are objects of $D$ with some extra property. But if $C$ is reflective in $D$, then we can turn this around and (by thinking of the left adjoint as a forgetful functor) think of objects of $D$ as objects of $C$ with (if we're lucky) some extra structure or (in any case) some extra stuff.

This can always be made to work by brute force, but sometimes there is something insightful about it. For example, a metric space is a complete metric space equipped with a dense subset. Or, a possibly nonunital ring is a unital ring equipped with a unital homomorphism to the ring of integers.

A terminological remark

A few sources (such as Categories Work) do not require a reflective subcategory to be full. However, in light of the fact that non-full subcategories are not invariant under equivalence, consideration of non-full reflective subcategories seems of limited usefulness. The general consensus among category theorists nowadays seems to be that “reflective subcategory” implies fullness.

References

• Jiri Adamek, Jiří Rosický, Locally presentable and accessible categories London Mathematical Society Lecture Note Series 189

• Springer eom: reflective subcategory

• cf. the notion of $Q^{\circ }$-category in the entry Q-category

The relation of exponential ideals to reflective subcategories is discussed in section A4.3.1 of

• Peter Johnstone, Sketches of an Elephant

Reflective and coreflective subcategories of presheaf categories are discussed in

• R. Bashir, J. Velebil, Simultaneously reflective and coreflective subcategories of presheaves, Theory and Applicaitons of Physics, Vol 10. No. 16. (2002) (pdf).