nLab bisimplicial set



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 bisimplicial set is a bisimplicial object in Set.





For X ,X_{\bullet,\bullet} a bisimplicial set, its diagonal is the simplicial set that is the precomposition with (Id,Id):Δ opΔ op×Δ op(Id, Id) \colon \Delta^{op} \to \Delta^{op} \times \Delta^{op} with the diagonal functor on the opposite of the simplex category, i.e. the simplicial set with components

d(X) nX n,n. d(X)_n \;\coloneqq\; X_{n,n} \,.


The realization |X||X| of a bisimplicial set X ,X_{\bullet,\bullet} is the simplicial set that is given by the coend

|X|= [n]ΔX n,*×Δ[n] |X| = \int^{[n] \in \Delta} X_{n, *} \times \Delta[n]

in sSet.


(diagonal is realization)

For XX a bisimplicial set, its diagonal d(X)d(X) is (isomorphic to) its realization |X||X|:

|X|d(X). |X| \simeq d(X) \,.

This is for instance exercise 1.6 in in chapter 4 Goerss-Jardine. For a derivation see the examples at homotopy colimit.


(diagonal is homotopy colimit)

The diagonal of a bisimplicial set X ,X_{\bullet,\bullet} is also (up to weak equivalence) the homotopy colimit of XX regarded as a simplicial diagram in the model structure on simplicial sets

diagXhocolim(X:Δ opsSet Quillen). diag X \simeq hocolim (X : \Delta^{op} \to sSet_{Quillen}) \,.

This appears for instance as theorem 3.6 in (Isaacson).


This follows with the above equivalence to the coend diagX [k]ΔΔ[k]X kdiag X \simeq \int^{[k] \in \Delta} \Delta[k] \cdot X_k and general expression of homotopy colimits by coends (as discussed there) in terms of the Quillen bifunctor

Δ()():[Δ,sSet Quillen] Reedy×[Δ,sSet Quillen] ReedysSet Quillen \int^\Delta (-) \cdot (-) : [\Delta, sSet_{Quillen}]_{Reedy} \times [\Delta, sSet_{Quillen}]_{Reedy} \to sSet_{Quillen}

in Reedy model structures (as discussed there) by using that Δ[]:ΔsSet Quillen\Delta[-] : \Delta \to sSet_{Quillen} is a Reedy cofibrant resultion of the point in [Δ,sSet Quillen][\Delta, sSet_{Quillen}] and that every object in [Δ op,sSet Quillen] Reedy[\Delta^{op}, sSet_{Quillen}]_{Reedy} is cofibrant.


(degreewise weak equivalences)

Let X,Y:Δ op×Δ opSetX,Y : \Delta^{op} \times \Delta^{op} \to Set be bisimplicial sets. A morphism f:XYf : X \to Y which is degreewise in one argument a weak equivalence f n,:X(n,)Y(n,)f_{n,\bullet} : X(n,\bullet) \to Y(n,\bullet) induces a weak equivalence d(f):d(X)d(Y)d(f) : d(X) \to d(Y) of the associated diagonal simplicial sets (with respect to the standard model structure on simplicial sets).


This is prop 1.9 in chapter 4 of

  • Goerss-Jardine, Simplicial Homotopy Theory (dvi)

Total décalage and total simplicial sets

There is a functor called ordinal sum (see also at simplex category)

+:Δ op×Δ opΔ op. + : \Delta^\op \times \Delta^{op} \to \Delta^{op} \,.
+:[k],[l][k+l+1]. + : [k], [l] \mapsto [k+l+1] \,.

This induces an adjoint triple

ssSet+ *+ *+ !sSet. ssSet \stackrel{\overset{+_!}{\longrightarrow}}{\stackrel{\overset{+^*}{\longleftarrow}}{\underset{+_*}{\longrightarrow}}} sSet \,.


  • T+ *T \coloneqq +_* is called the total simplicial set functor or Artin-Mazur codiagonal (we will use the first of these as codiagonal also has another accepted meaning, see codiagonal);

  • Dec+ *Dec \coloneqq +^* is called the total décalage functor (inside which is plain décalage);


TT preserves degreewise weak equivalences of simplicial sets.


For XX any bisimplicial set

These statements are for instance in (CegarraRemedios) and (Stevenson). They may be considered as a non-additive versions of the Eilenberg-Zilber theorem.


By prop. and the usual Eilenberg-Zilber theorem it follows that under forming chain complexes for simplicial homology, total simplicial sets correspond to total complexes of double complexes.


After geometric realization these spaces are even related by a homeomorphism.

(This seems to be due to Berger and Hübschmann, but related results were known to Zisman as they are so mentioned by Cordier in his work on homotopy limits.)


The simplicial classifying space-functor for simplicial groups

W¯:sGrpsSet * \overline{W} \;\colon\; sGrp \longrightarrow sSet_*

is the composite

W¯:sGrpBsGrpdNssSetTsSet. \bar W \colon sGrp \stackrel{\mathbf{B}}{\longrightarrow} sGrpd \stackrel{N}{\longrightarrow} ssSet \stackrel{T}{\longrightarrow} sSet \,.

We have the following explicit formula for TXT X, attributed to John Duskin:


For XX a bisimplicial set the total simplicial set TXT X is in degree nn the equalizer

(TX) n i=0 nX i,ni i=0 n1X i,ni1 (T X)_n \to \prod_{i = 0}^n X_{i, n-i} \stackrel{\longrightarrow}{\longrightarrow} \prod_{i = 0}^{n-1} X_{i, n-i-1}

where the components of the two morphisms on the right are

i=0 nX i,nip iX i,nid 0 vX i,ni1 \prod_{i = 0}^n X_{i,n-i} \stackrel{p_i}{\to} X_{i, n-i} \stackrel{d_0^v}{\to} X_{i, n-i-1}


i=0 nX i,nip i+1X i+1,ni1d i+1 hX i,ni1. \prod_{i = 0}^n X_{i,n-i} \stackrel{p_{i+1}}{\to} X_{i+1,n-i-1} \stackrel{d_{i+1}^h}{\to} X_{i,n-i-1} \,.

The face maps d i:(TX) n(TX) n1d_i : (T X)_n \to (T X)_{n-1} are given by

d i=(d i vp 0,d i1 vp 1,,d 1 vp i1,d i hp i+1,d i hp i+2,,d i hp n) d_i = (d_i^v p_0, d_{i-1}^v p_1, \cdots, d_1^v p_{i-1}, d_i^h p_{i+1}, d_i^h p_{i+2}, \cdots, d_i^h p_n )

and the degeneracy maps are given by

s i=(s i vp 0,s i1 vp 1,,s 0 vp i,s i hp i+1,,s i hp n). s_i = (s_i^v p_0, s_{i-1}^v p_1, \cdots, s_0^v p_i, s_i^h p_{i+1}, \cdots, s_i^h p_n) \,.

The (DecT)(Dec \dashv T)-adjunction unit η:XTDecX\eta : X \to T Dec X is given in degree nn by

η:x(s 0(x),s 1(x),,s n(x)). \eta : x \mapsto (s_0(x), s_1(x), \cdots, s_n(x)) \,.

Geometric realization

See geometric realization of simplicial topological spaces.

Model structures

There are various useful model category structures on the category of bisimplicial sets.

Induced from the diagonal

There is an adjunction

(Ldiag):ssSetdiagLsSet. (L \dashv diag) : ssSet \stackrel{\overset{L}{\longleftarrow}}{\underset{diag}{\longrightarrow}} sSet \,.

The transferred model structure along this adjunction of the standard model structure on simplicial sets exists and with respect to it the above Quillen adjunction is a Quillen equivalence.

This is due to (Moerdijk 89)

Induced from codiagonal TT.

The transferred model structure on ssSetssSet along the total simplicial set functor TT exists. And for it

(DecT):ssSetTDecsSet (Dec \dashv T) : ssSet \stackrel{\overset{Dec}{\longleftarrow}}{\underset{T}{\longrightarrow}} sSet

is a Quillen equivalence.


Every diag-fibration is also a TT-fibration.

This is (CegarraRemedios, theorem 9).

Remark on notation

There are two uses of W¯\bar W in this area, one is as used in (CegarraRemedios) where it is used for the codiagonal (sometimes denoted “\nabla” or TT as above), the other is for the classifying space functor for a simplicial group. This latter is not only the older of the two uses, but also comes with a related WW construction. The relationship between the two is that given a simplicial group or simplicially enriched groupoid, GG, applying the nerve functor in each dimension gives a bisimplicial set and W¯G=TNerG\bar{W}G = T Ner G. Because of this, some care is needed when using these sources.

Bisimplicial abelian groups


Let A,B:Δ op×Δ opAbA,B : \Delta^{op} \times \Delta^{op} \to Ab be bisimplicial abelian groups. A morphism f:ABf : A \to B which is degreewise in one argument a weak equivalence f n,:A(n,)B(n,)f_{n,\bullet} : A(n,\bullet) \to B(n,\bullet) induces a weak equivalence d(f):d(A)d(B)d(f) : d(A) \to d(B) of the associated diagonal complexes.


This is Lemma 2.7 in chapter 4 of (GoerssJardine)


Some standard material is for instance in

The total simplicial set functor goes back to

The diagonal, total décalage and total simplicial set constructions are discussed in

The diagonal-induced model structure on ssSetssSet is discussed in

  • Ieke Moerdijk, Bisimplicial sets and the group completion theorem in Algebraic K-Theory: Connections with Geometry and Topology, pp 225–240. Kluwer, Dordrecht (1989)

On the behaviour of fibrations under geometric realization of bisimplicial sets:

Discussion of the simplicial classifying space-construction W¯\overline{W} respecting fibrant objects is in Fact 2.8 of:

Last revised on October 14, 2023 at 06:23:45. See the history of this page for a list of all contributions to it.