nLab category of simplices

Categories of simplices


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Basic facts


Categories of simplices


For XX a simplicial set its category of simplices is the category whose objects are the simplices in XX and whose morphisms are maps between these, as simplices in XX.

In particular the subcategory on the non-degenerate simplices has a useful interpretation: it is the poset of subsimplex inclusions whose nerve is the barycentric subdivision of XX, at least if every non-degenerate simplex in XX comes from a monomorphism Δ nX\Delta^n \to X, as for a simplicial complex.


Let XX \in sSet be a simplicial set.


The category of simplices of XX is equivalently (in increasing order of explicitness)

  • the category of elements of the presheaf X :Δ opSetX_\bullet : \Delta^{op} \to Set;

  • the comma category (ΔX)(\Delta\downarrow X), where Δ\Delta denotes the Yoneda embedding [n]Δ n[n] \mapsto \Delta^n.

  • the category whose objects are homomorphisms of simplicial sets c:Δ nXc : \Delta^n \to X from a standard simplicial simplex Δ n\Delta^n to XX, and whose morphisms ccc \to c' are morphisms f:Δ nΔ nf : \Delta^n \to \Delta^{n'} in the simplex category Δ\Delta such that the diagram

    Δ n f Δ n c c X \array{ \Delta^n && \stackrel{f}{\longrightarrow} && \Delta^{n'} \\ & {}_{c}\searrow && \swarrow_{c'} \\ && X }



An nn-simplex xX nx\in X_n is said to be non-degenerate if it is not in the image of any degeneracy map.


(ΔX) nondeg(ΔX) (\Delta\downarrow X)_{nondeg}\hookrightarrow (\Delta\downarrow X)

for the (non-full) subcategory on the non-degenerate simplices. Notice that a morphism of ΔX\Delta\downarrow X with source a non-degenerate simplex of XX is necessary a monomorphism.

This is called the category of non-degenerate simplices.


If every non-degenerate simplex in XX comes from a monomorphism Δ nX\Delta^n \to X, then the nerve N((ΔX) nondeg)N((\Delta \downarrow X)_{nondeg}) is also called the barycentric subdivision of XX.

See at barycentric subdivision – Relation to the category of simplices.




If XX has the property that every face of every non-degenerate simplex is again non-degenerate, then the inclusion of the category of non-degenerate simplices (ΔX) nondeg(ΔX)(\Delta \downarrow X)_{nondeg} \hookrightarrow (\Delta \downarrow X) has a left adjoint and is hence a reflective subcategory.


The category of simplices is a Reedy category.


Write (ΔX)sSet(\Delta \downarrow X) \to sSet for the canonical functor that sends (Δ nX)(\Delta^n \to X) to Δ n\Delta^n.


The colimit over the functor (ΔX)sSet(\Delta \downarrow X) \to sSet is XX itself

Xlim((ΔX)sSet) X \simeq \underset{\to}{\lim}((\Delta \downarrow X) \to sSet)

By the co-Yoneda lemma.

In the textbook literature this appears for instance as (Hovey, lemma 3.1.3).


A colimit-preserving functor F:sSetCF\colon sSet \to C is uniquely determined by its action on the standard simplices:

F(X)colim (ΔX)F(Δ ). F(X) \cong colim_{(\Delta\downarrow X)} F(\Delta^\bullet).

Important colimit-preserving functors out of sSet include

The nerve and subdivision

Let N:N\colon Cat \to sSet denote the simplicial nerve functor on categories.


The functor sSetsSetsSet \to sSet that assigns barycentric subdivision, def. ,

XN(ΔX) X\mapsto N(\Delta\downarrow X)

preserves colimits.


An nn-simplex of N(ΔX)N(\Delta\downarrow X) is determined by a string of n+1n+1 composable morphisms

Δ k nΔ k 0 \Delta^{k_n} \to \dots\to \Delta^{k_0}

along with a map Δ k 0X\Delta^{k_0} \to X, i.e. an element of X k 0X_{k_0} Thus, each the functor XN(ΔX) nX\mapsto N(\Delta\downarrow X)_n from SSetSetSSet \to Set is a coproduct of a family of “evaluation” functors. Since evaluation preserve colimits, coproducts commute with colimits, and colimits in SSetSSet are levelwise, the statement follows.

Therefore, the simplicial set N(ΔX)N(\Delta\downarrow X) itself can be computed as a colimit over the category (ΔX)(\Delta\downarrow X) of the simplicial sets N(ΔΔ n)N(\Delta\downarrow \Delta^n).


The terminology “category of simplices” is attributed by Hovey 1999 to:

Textbook account:

Homotopy finality of the non-degenerate simplices:

More on barycentric subdivision:

  • J. F. Jardine, Section 2 of: Simplicial approximation, Theory and Applications of Categories, 12 2 (2004) 34-72 [tac:12-02]

Last revised on August 19, 2022 at 21:48:43. See the history of this page for a list of all contributions to it.