nLab
simplicial set

Content

Idea

Simplicial sets generalize the idea of simplicial complexes: a simplicial set is like a combinatorial space built up out of gluing abstract simplices to each other. Equivalently, it is an object equipped with a rule for how to consistently map the objects of the simplex category into it.

More concretely, a simplicial set SS is a collection of sets S nS_n for nn \in \mathbb{N}, so that elements in S nS_n are to be thought of as nn-simplices, equipped with a rule that says:

  • which (n1)(n-1)-simplices in S n1S_{n-1} are faces of which elements of S nS_n;
  • which (n+1)(n+1)-simplices are thin in that they are really just nn-simplices regarded as degenerate (n+1)(n+1)-simplices.

One of the main uses of simplicial sets is as combinatorial models for the (weak) homotopy type of topological spaces. They can also be taken as models for ∞-groupoids. This is encoded in the model structure on simplicial sets. For more reasons why simplicial sets see MathOverflow here.

Definition

Definition

A simplicial set is a presheaf on the simplex category Δ\Delta, that is, a functor X:Δ opSetsX : \Delta^{op} \to Sets, equivalently a simplicial object in the category Set of sets.

With the standard morphisms of presheaves as morphisms, simplicial sets form the category sSet (also called SSetSSet or sSetsSet).

More explicitly this means the following.

Definition

A simplicial set SsSetS \in sSet is

  • for each nn \in \mathbb{N} a set S nSetS_n \in Set – the set of nn-simplices;

  • for each injective map δ i:n1¯n¯\delta_i : \overline{n-1} \to \overline{n} of totally ordered sets n¯{0<1<<n}\bar n \coloneqq \{ 0 \lt 1 \lt \cdots \lt n \}

    a function d i:S nS n1d_i : S_{n} \to S_{n-1} – the iith face map on nn-simplices;

  • for each surjective map σ i:n+1¯n¯\sigma_i : \overline{n+1} \to \bar n of totally ordered sets

    a function σ i:S nS n+1\sigma_i : S_{n} \to S_{n+1} – the iith degeneracy map on nn-simplices;

such that these functions satisfy the simplicial identities.

Remarks

Simplicial sets as spaces built of simplices

  • The definition is to be understood from the point of view of space and quantity: a simplicial set is a space characterized by the fact that and how it may be probed by mapping standard simplices into it: the set S nS_n assigned by a simplicial set to the standard nn-simplex [n][n] is the set of nn-simplices in this space, hence the way of mapping a standard nn-simplex into this spaces.

  • For SS a simplicial set, the face map

    d i:=S(δ i):S nS n1 d_i := S(\delta^i): S_n \rightarrow S_{n-1}

    is dual to the unique injection δ i:[n1][n]\delta^i : [n-1] \rightarrow [n] in the category Δ\Delta whose image omits the element i[n]i \in [n].

  • Similarly, the degeneracy map

    s i:=S(σ i):S nS n+1 s_i := S(\sigma^i) : S_n \rightarrow S_{n+1}

    is dual to the unique surjection σ i:[n+1][n]\sigma^i : [n+1] \rightarrow [n] in Δ\Delta such that i[n]i \in [n] has two elements in its preimage.

  • The maps δ i\delta^i and σ i\sigma^i satisfy certain obvious relations – the simplicial identities – dual to those spelled out at simplex category.

Visualisation

(based on cubical set)

The face maps go from sets S n+1S_{n+1} of (n+1)(n+1)-dimensional simplices to the corresponding set S nS_{n} of nn-dimensional simplices and can be thought of as sending each simplex in the simplicial set to one of its faces, for instance for n=1n=1 the set S 2S_2 of 2-simplices would be sent in three different ways by three different face maps to the set of 11-simplices, for instance one of the face maps would send

( b F a c)( b a) \left( \array{ & & b \\ & \nearrow & \Downarrow^F & \searrow \\ a & & \rightarrow & & c } \right) \;\; \mapsto \;\; \left( \array{ & & b \\ & \nearrow \\ a } \right)

another one would send

( b F a c)(a c). \left( \array{ & & b \\ & \nearrow & \Downarrow^F & \searrow \\ a & & \rightarrow & & c } \right) \;\; \mapsto \;\; \left( \array{ a & & \rightarrow & & c } \right) \,.

On the other hand, the degeneracy maps go the other way round and send sets S nS_n of nn-simplices to sets S n+1S_{n+1} of (n+1)(n+1)-simplices by regarding an nn-simplex as a degenerate or “thin” (n+1)(n+1)-simplex in the various different ways that this is possible. For instance, again for n=1n=1, a degeneracy map may act by sending

(a f b)( b f Id Id a f b). \left( \array{ a &\stackrel{f}{\to}& b } \right) \;\; \mapsto \;\; \left( \array{ & & b \\ & \nearrow_f & \Downarrow^{Id} & \searrow^{Id} \\ a & & \stackrel{f}\to & & b } \right) \,.

Notice the IdId-labels, which indicate that the edges and faces labeled by them are “thin” in much the same way as an identity morphism is thin. They depend on lower dimensional features, (however notice however that a simplicial set by itself is not equipped with any notion of composition of simplices, nor really, therefore, of identities. See quasicategory for a kind of simplicial set which does have such notions and simplicial T-complex for more on the intuitions behind this idea of ‘’thinness’’).

Examples

Singular simplices (Yoneda embeddings)

Let [n][n] denote the object of Δ\Delta corresponding to the totally ordered set {0,1,2,,n}\{ 0, 1, 2,\ldots, n\}. Then the represented presheaf Δ(,[n])\Delta(-, [n]), typically written as Δ[n]\Delta[n] is an example of a simplicial set. In particular we have Δ[n] m=Hom Δ([m],[n])\Delta[n]_m=Hom_\Delta([m],[n]) and hence Δ[n] m\Delta[n]_m is a finite set with (n+m+1n)\binom{n+m+1}{n} elements.

By the Yoneda lemma, the nn-simplices of a simplicial set XX are in natural bijective correspondence to maps Δ[n]X\Delta[n] \rightarrow X of simplicial sets.

Nerve of a category

If CC is a small category, the nerve of CC is a simplicial set which we denote NCNC. If we intepret the poset [n][n] defined above as a category, we define the nn-simplices of NCNC to be the set of functors [n]C[n] \rightarrow C. Equivalently, the 00-simplices of NCNC are the objects of CC, the 11-simplices are the morphisms, and the nn-simplices are strings of nn composable arrows in CC. Face maps are given by composition (or omission, in the case of d 0d_0 and d nd_n) and degeneracy maps are given by inserting identity arrows.

Singular simplicial complex of a topological space

Recall from simplex category or geometric realization the standard functor ΔTop\Delta \to Top which sends [n]Δ[n] \in \Delta to the standard topological nn-simplex Δ n\Delta^n. This functor induces for every topological space XX the simplicial set

SX:[n]Hom Top(Δ n,X) S X : [n] \mapsto Hom_{Top}(\Delta^n, X)

called the simplicial singular complex of XX. This simplicial set is always a Kan complex and may be regarded as the fundamental ∞-groupoid of XX.

Following up on the idea of ‘’thinness’’, a singular simplex f:Δ nXf: \Delta^n \to X may be called thin if it factors through a retraction r:Δ nΛ i n1r: \Delta^n \to \Lambda^{n-1}_i to some horn of Δ n\Delta^n, then the well known Kan condition on SXS X can be strengthened to say that every horn in SXS X has a thin filler. This also helps to give some intuitive underpinning to the idea of thin element in this simplicial context.

Bar construction

For the moment see bar construction.

Properties

Classifying topos

Simplicial sets

The category of simplicial sets is a presheaf category, and so in particular a Grothendieck topos. In fact, it is the classifying topos of the theory of “intervals”, meaning totally ordered sets equipped with distinct top and bottom elements.

Specifically, if EE is a topos containing such an interval II, then we obtain a functor ΔE\Delta \to E sending [n][n] to the subobject

{(x 1,x 2,,x n)|x 1x 2x n}I n \{ (x_1,x_2,\dots,x_n) \;|\; x_1 \le x_2 \le \dots \le x_n \} \hookrightarrow I^n

The corresponding geometric realization/nerve adjunction ESet Δ opE \leftrightarrows Set^{\Delta^{op}} is the geometric morphism which classifies II.

The usual geometric realization into topological spaces cannot be obtained in this way precisely, since Top is not a topos. However, there are Top-like categories which are toposes, such as Johnstone's topological topos.

Cosimplicial sets

Similarly, also the category Set ΔSet^{\Delta} of cosimplicial sets is a classifying topos: for inhabited linear orders. See at classifying topos the section For (inhabited) linear orders.

As models in homotopy theory

(…) homotopy theory (…) Kan complex (…) quasi-category (…)

Relation to dendroidal sets

For the moment see at dendroidal set the section Relation to simplicial sets

Variants

  • A symmetric set is a simplicial set equipped with additional transposition maps t i n:X nX nt^n_i: X_n \to X_n for i=0,,n1i=0,\ldots,n-1. These transition maps generate an action of the symmetric group on X nX_n and satisfy certain commutation relations with the face and degeneracy maps.

References

A pedagogical introduction to simplicial sets is

  • Greg Friedman, An elementary illustrated introduction to simplicial sets (arXiv:0809.4221)

A very clear and explicit exposition on the basics of simplicial sets is

  • Emily Riehl, A leisurely introduction to simplicial sets, 2008, 14 pages (pdf).

Another clear exposition is in the classic

A useful (if old) survey article is:

  • Edward B. Curtis, Simplicial homotopy theory, Advances in Math., 6 (1971) 107 – 209 MR279808 doi

More advanced treatments include

Some more facts about homotopical aspects of simplicial sets are discussed in section 2 of

category: topology

Revised on March 31, 2014 01:06:09 by Urs Schreiber (82.113.99.73)