nLab core


This entry is about the concept in category theory. For the core of a ring see there.



For 𝒞\mathcal{C} any category or, more generally, (n,r)-category, its core is the maximal groupoid inside it, generally the maximal sub-(n,0)-category, hence the maximal \infty -groupoid inside it.

Sincefunctor out of a groupoid 𝒢\mathcal{G}, and generally an (n,r)-functor out of an \infty -groupoid, necessarily takes values in isomorphisms or generally in equivalences, hence in the core of its codomain,

Cat n,r(𝒢,𝒞)Grpd n(𝒢,core(𝒞)), Cat_{n,r} \big( \mathcal{G} ,\, \mathcal{C} \big) \;\; \simeq \;\; Grpd_{n} \big( \mathcal{G} ,\, core(\mathcal{C}) \big) \,,

the core construction is the right adjoint to the full inclusion of groupoids among categories (generally: (n,0)-categories among (n,r)-categories):

Grpd ncoreCat n,r. Grpd_n \underoverset {\underset{core}{\longleftarrow}} {\hookrightarrow} {\bot} Cat_{n,r} \,.



For CC \in Cat a category, its core core(C)core(C) \in Grpd
is the groupoid which is the maximal sub-groupoid of CC: the subcategory consisting of all objects of CC but with morphisms only the isomorphisms of CC.

This construction extends to a 1-functor

Core:CatGrpd. Core \colon Cat \to Grpd \,.

We usually think of a groupoid as a special kind of category, but we can also think of a category as a groupoid equipped with additional morphisms. (This is possible because Grpd is a reflective subcategory of Cat.) One level decategorified, we usually think in the opposite way: a poset is a set equipped with a partial order, but we can also think of a set as a special kind of poset (specifically, a symmetric one).


  • Given a preordered set, regarded as a category, taking its core is the same as partitioning the set into equivalence classes of the preorder.

  • A combinatorial species is defined as a presheaf, that is, a contravariant functor to Set, on the core of FinSet.

Every groupoid has a contravariant functor to itself. It preserves the objects and sends the arrows to their inverses.


Universal property


The core-functor of def. is right adjoint to the full subcategory-inclusion U:GrpdCatU \colon Grpd \to Cat of groupoids into categories.

GrpdcoreCat. Grpd \underoverset {\underset{core}{\longleftarrow}} {\hookrightarrow} {\bot} Cat \,.

Given a category CC and a groupoid AA, a functor

AC A \to C

(hence a functor out of the underlying category U(A)U(A) of AA) has to send isomorphisms to isomorphisms, hence has to send every morphism of AA to an isomorphism in CC. This means that it factors through the core-inclusion

ACore(C)C. A \to Core(C) \to C \,.

The left adjoint to U:GrpdCatU \colon Grpd \to Cat is the localization functor that universally inverts every morphism in CC. On nerves this is Kan fibrant replacement.

Variations and generalizations


The core of a dagger category consists of its unitary isomorphisms only. This is why, for example, it makes sense to think of Hilb either as a category whose morphisms are linear maps bounded by 11 or as a dagger category whose morphisms are all linear maps; either way, the core is the same (invertible linear maps of norm exactly 11).

Higher categories

The core of an nn-category is the nn-groupoid consisting only of equivalences at each level; the core of an \infty-category is similarly an \infty-groupoid: the core of a quasicategory is the maximal Kan complex inside it.

For more on this see also at category object in an (infinity,1)-category.


Discussion in the generality of ( , n ) (\infty,n) -categories:

Last revised on October 23, 2022 at 09:04:30. See the history of this page for a list of all contributions to it.