nLab replete subcategory

Replete subcategories


Category theory

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higher category theory

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1-categorical presentations

Replete subcategories


A subcategory DD of a strict category CC is called replete if it respects isomorphism of morphisms in the arrow category of CC. It is a subcategory for which the property of (strictly) belonging to it respects the principle of equivalence of categories.


A subcategory DD of CC is replete if for any object xx in DD and any isomorphism f:xyf\colon x\cong y in CC, both yy and ff are also in DD. Equivalent ways to state this include:

  • If fDf \in D and fgf \cong g in the arrow category Arr(C)Arr(C), then gDg \in D.

  • The inclusion DCD\hookrightarrow C is an isofibration.

Repletion and replete images

Since repleteness is a “closure condition,” the intersection of any collection of replete subcategories is again replete. Therefore, any subcategory is contained in a smallest replete subcategory, called its repletion. We can construct the repletion repl(D)repl(D) of DCD\subset C explicitly as follows:

  • its objects are those objects of CC which admit an isomorphism to some object of DD, and
  • its morphisms are those morphisms of CC which can be written as a composite of morphisms in DD and isomorphisms in CC.

Repleteness and repletions are most often applied to full subcategories, in which case the repletion is simply the full subcategory of CC determined by those objects which are isomorphic to some object of DD. In particular, in this case, the repletion is equivalent to DD. More generally, we can say:


The inclusion Drepl(D)D\hookrightarrow repl(D) is an equivalence if and only if the inclusion DCD\hookrightarrow C is pseudomonic.


The inclusion of a replete subcategory is always pseudomonic, so if Drepl(D)D\hookrightarrow repl(D) is an equivalence, and in particular full and faithful, then DCD\hookrightarrow C must also be pseudomonic.

Conversely, if DCD\hookrightarrow C is pseudomonic, then every morphism of repl(D)repl(D) can be written as gfh 1g f h^{-1} for some morphism ff in DD and isomorphisms gg and hh in CC. If the domain and codomain are in DD, then since DCD\hookrightarrow C is pseudomonic, both gg and hh must also be in DD, hence such a morphism is itself necessarily in DD. Thus the inclusion Drepl(D)D\hookrightarrow repl(D) is full. It is clearly faithful and essentially surjective, so it is an equivalence.

The replete image of a functor is the repletion of its image. The replete full image of a functor is the repletion of its full image, i.e. the full subcategory of its target determined by those objects isomorphic to some object in its image. See also essential image.

Higher-categorical versions

More generally, for n0n\geq 0, a subcategory DD of an nn-category CC is replete if all equivalences in CC whose either source or target is in DD are themselves in DD. In particular, all kk-cells equivalent in CC to some kk-cell in DD are themselves in DD (because they are sources of some equivalence in DD). Then replete subcategories of 11-categories are those which are replete in this sense.

Here we are using the weakest notion of equivalence; one could also talk about kk-replete nn-subcategories if every kk-equivalences in CC belongs to DD if either its source or target does. Then using the usual definition of ‘category’ as a model for 11-categories, replete subcategories are already those which are 00-replete (indeed, 00-equivalence is isomorphism in this model); but using a model of 11-categories in which hom-sets are setoids (as is common, for example, in type-theoretic foundations), we must insist on 11-repleteness.

Note that a subcategory DD of an nn-category CC is (k1)(k-1)-replete if and only if:

  • for every (nk)(n-k)-cell in DD all (nk)(n-k)-cells (k1)(k-1)-equivalent to an (nk)(n-k)-cell in DD are themselves in CC, and
  • for any two (nk)(n-k)-cells x,yx,y the hom kk-category hom(x,y)hom(x,y) is (k2)(k-2)-replete.

Last revised on July 13, 2023 at 16:49:12. See the history of this page for a list of all contributions to it.