simplicial object


Category theory

Homotopy theory

homotopy theory, (∞,1)-category theory, homotopy type theory

flavors: stable, equivariant, rational, p-adic, proper, geometric, cohesive, directed

models: topological, simplicial, localic, …

see also algebraic topology



Paths and cylinders

Homotopy groups

Basic facts




A simplicial object XX in a category CC is an simplicial set internal to CC: a collection {X n} n\{X_n\}_{n \in \mathbb{N}} of objects in CC that behave as if X nX_n were an object of nn-dimensional simplices internal to CC equipped with maps between these space that assign faces and degenerate simplices.

For instance, and there is a longer list further down this page, a simplicial object in GrpsGrps is a collection {G n} n\{G_n\}_{n\in \mathbb{N}} of groups, together with face and degeneracy homomorphisms between them. This is just a simplicial group. We equally well have other important instances of the same idea, when we replace GrpsGrps by other categories, or higher categories.


A simplicial object in a category CC is a functor Δ opC\Delta^{op} \to C, where Δ\Delta is the simplicial indexing category.

More generally, a simplicial object in an (∞,1)-category is an (∞,1)-functor Δ opC\Delta^{op} \to C.

A cosimplicial object in CC is similarly a functor out of the opposite category, ΔC\Delta \to C.

Accordingly, simplicial and cosimplicial objects in CC themselves form a category in an obvious way, namely the functor category [Δ op,C][\Delta^{op},C] and [Δ,C][\Delta,C], respectively.


A simplicial object XX in CC is often specified by the objects, X nX_n, which are the images under XX, of the objects [n][n] of Δ\Delta, together with a description of the face and degeneracy morphisms, d id_i and s js_j, which must satisfy the simplicial identities.


Category of simplicial objects

For DD a category, we write D Δ opD^{\Delta^{op}} for the functor category from Δ op\Delta^{op} to DD: its category of simplicial objects.


Let DD be a category with all limits and colimits. This implies that it is tensored over Set

:D×SetD. \cdot : D \times Set \to D \,.

This induces a functor

Δ op:D Δ op×sSetD Δ op \cdot^{\Delta^{op}} : D^{\Delta^{op}} \times sSet \to D^{\Delta^{op}}

which we shall also write just “\cdot”.

For X,YD Δ opX,Y \in D^{\Delta^{op}} write

D Δ op(X,Y):=Hom D Δ op(XΔ[],Y)sSet D^{\Delta^{op}}(X,Y) := Hom_{D^{\Delta^{op}}}(X \cdot \Delta[\bullet], Y) \in sSet

and for X,Y,ZD Δ opX,Y,Z \in D^{\Delta^{op}} let

D Δ op(X,Y)×D Δ op(Y,Z)D Δ op(X,Z) D^{\Delta^{op}}(X,Y) \times D^{\Delta^{op}}(Y,Z) \to D^{\Delta^{op}}(X,Z)

be given in degree nn by

(XΔ[n]Y,YΔ[n]Z)(XΔ[n]XΔ[n]×Δ[n]YΔ[n]Z). (X \cdot \Delta[n] \to Y, Y \cdot \Delta[n] \to Z) \mapsto ( X \cdot \Delta[n] \to X \cdot \Delta[n]\times \Delta[n] \to Y \cdot \Delta[n] \to Z) \,.

With the above definitions D Δ opD^{\Delta^{op}} becomes an sSet-enriched category which is both tensored as well as cotensored over sSetsSet.


We may regard the category of cosimplicial objects D ΔD^{\Delta} as an sSetsSet-enriched category using the above enrichment by identifying

D Δ(D op Δ op) op. D^{\Delta} \simeq ({D^{op}}^{\Delta^{op}})^{op} \,.


  • Peter May, Simplicial objects in algebraic topology , University of Chicago Press, 1967, (djvu)

Revised on February 10, 2015 10:43:51 by Urs Schreiber (