# Simplicial set

In mathematics, a simplicial set is a construction in categorical homotopy theory that is a purely algebraic model of the notion of a "well-behaved" topological space. Historically, this model arose from earlier work in combinatorial topology and in particular from the notion of simplicial complexes. Simplicial sets are used to define quasi-categories, a basic notion of higher category theory.

## Motivation

A simplicial set is a categorical (that is, purely algebraic) model capturing those topological spaces that can be built up (or faithfully represented up to homotopy) from simplices and their incidence relations. This is similar to the approach of CW complexes to modeling topological spaces, with the crucial difference that simplicial sets are purely algebraic and do not carry any actual topology (this will become clear in the formal definition).

To get back to actual topological spaces, there is a geometric realization functor which turns simplicial sets into compactly generated Hausdorff spaces. Most classical results on CW complexes in homotopy theory have analogous versions for simplicial sets which generalize these results. While algebraic topologists largely continue to prefer CW complexes, there is a growing contingent of researchers interested in using simplicial sets for applications in algebraic geometry where CW complexes do not naturally exist.

## Intuition

Simplicial sets can be viewed as a higher-dimensional generalization of directed multigraphs. A simplicial set contains vertices (known as "0-simplices" in this context) and arrows ("1-simplices") between some of these vertices. Two vertices may be connected by several arrows, and directed loops that connect a vertex to itself are also allowed. Unlike directed multigraphs, simplicial sets may also contain higher simplices. A 2-simplex, for instance, can be thought of as a two-dimensional "triangular" shape bounded by an ordered list of three vertices A, B, C and three arrows f:AB, g:BC and h:AC. In general, an n-simplex is an object made up from an ordered list of n+1 vertices (which are 0-simplices) and n+1 faces (which are (n-1)-simplices). The vertices of the i-th face are the vertices of the n-simplex minus the i-th vertex. The vertices of a simplex need not be distinct and a simplex is not determined by its vertices and faces: two different simplices may share the same list of faces (and therefore the same list of vertices).

Simplicial sets should not be confused with abstract simplicial complexes, which generalize simple undirected graphs rather than directed multigraphs.

Formally, a simplicial set X is a collection of sets Xn, n=0,1,2,..., together with certain maps between these sets: the face maps dn,i:XnXn-1 (n=1,2,3,... and 0≤in) and degeneracy maps sn,i:XnXn+1 (n=0,1,2,... and 0≤in). We think of the elements of Xn as the n-simplices of X. The map dn,i assigns to each such n-simplex its i-th face, the face "opposite to" (i.e. not containing) the i-th vertex. The map sn,i assigns to each n-simplex the degenerate (n+1)-simplex which arises from the given one by duplicating the i-th vertex. This description implicitly requires certain consistency relations among the maps dn,i and sn,i. Rather than requiring these simplicial identities explicitly as part of the definition, the short and elegant modern definition uses the language of category theory.

## Formal definition

Let Δ denote the simplex category. The objects of Δ are nonempty linearly ordered sets of the form

[n] = {0, 1, ..., n}

with n≥0. The morphisms in Δ are (non-strictly) order-preserving functions between these sets.

A simplicial set X is a contravariant functor

X: Δ → Set

where Set is the category of small sets. (Alternatively and equivalently, one may define simplicial sets as covariant functors from the opposite category Δop to Set.) Simplicial sets are therefore nothing but presheafs on Δ.

Alternatively, one can think of a simplicial set as a simplicial object (see below) in the category Set, but this is only different language for the definition just given. If we use a covariant functor X: Δ → Set instead of a contravariant one, we arrive at the definition of a cosimplicial set.

Simplicial sets form a category, usually denoted sSet, whose objects are simplicial sets and whose morphisms are natural transformations between them. There is a corresponding category for cosimplicial sets as well, denoted by cSet.

## Face and degeneracy maps

The simplex category Δ is generated by two particularly important families of morphisms (maps), whose images under a given simplicial set functor are called face maps and degeneracy maps of that simplicial set.

The face maps of a simplicial set are the images in that simplicial set of the morphisms ${\displaystyle \delta ^{0},\dotsc ,\delta ^{n}\colon [n-1]\to [n]}$, where ${\displaystyle \delta ^{i}}$ is the only injection ${\displaystyle [n-1]\to [n]}$ that "misses" ${\displaystyle i}$. Let us denote these face maps by ${\displaystyle d_{0},\dotsc ,d_{n}}$ respectively.

The degeneracy maps of a simplicial set are the images in that simplicial set of the morphisms ${\displaystyle \sigma ^{0},\dotsc ,\sigma ^{n}\colon [n+1]\to [n]}$, where ${\displaystyle \sigma ^{i}}$ is the only surjection ${\displaystyle [n+1]\to [n]}$ that "hits" ${\displaystyle i}$ twice. Let us denote these degeneracy maps by ${\displaystyle s_{0},\dotsc ,s_{n}}$ respectively.

The defined maps satisfy the following simplicial identities:

1. di dj = dj−1 di if i < j
2. di sj = sj−1 di if i < j
3. di sj = id if i = j or i = j + 1
4. di sj = sj di−1 if i > j + 1
5. si sj = sj+1 si if ij.

## Examples

Given a partially ordered set (S,≤), we can define a simplicial set NS, the nerve of S, as follows: for every object [n] of Δ we set NS([n]) = hompo-set( [n] , S), the order-preserving maps from [n] to S. Every morphism φ:[n]→[m] in Δ is an order preserving map, and via composition induces a map NS(φ) : NS([m]) → NS([n]). It is straightforward to check that NS is a contravariant functor from Δ to Set: a simplicial set.

Concretely, the n-simplices of the nerve NS, i.e. the elements of NSn=NS([n]), can be thought of as ordered length-(n+1) sequences of elements from S: (a0a1 ≤ ... ≤ an). The face map di drops the i-th element from such a list, and the degeneracy maps si duplicates the i-th element.

A similar construction can be performed for every category C, to obtain the nerve NC of C. Here, NC([n]) is the set of all functors from [n] to C, where we consider [n] as a category with objects 0,1,...,n and a single morphism from i to j whenever ij.

Concretely, the n-simplices of the nerve NC can be thought of as sequences of n composable morphisms in C: a0a1→...→an. (In particular, the 0-simplices are the objects of C and the 1-simplices are the morphisms of C.) The face map d0 drops the first morphism from such a list, the face map dn drops the last, and the face map di for 0<i<n composes the (i-1)st and ith morphisms. The degeneracy maps si lengthen the sequence by inserting an identity morphism at position i.

We can recover the poset S from the nerve NS and the category C from the nerve NC; in this sense simplicial sets generalize posets and categories.

Another important class of examples of simplicial sets is given by the singular set SY of a topological space Y. Here SYn consists of all the continuous maps from the standard topological n-simplex to Y. The singular set is further explained below.

## The standard n-simplex and the category of simplices

The standard n-simplex, denoted Δn, is a simplicial set defined as the functor homΔ(-, [n]) where [n] denotes the ordered set {0, 1, ... ,n} of the first (n + 1) nonnegative integers. In many texts, it is written instead as hom([n],-) where the homset is understood to be in the opposite category Δop.[1]

The geometric realization |Δn| is just defined to be the standard topological n-simplex in general position given by

${\displaystyle |\Delta ^{n}|=\{(x_{0},\dots ,x_{n})\in \mathbb {R} ^{n+1}:0\leq x_{i}\leq 1,\sum x_{i}=1\}.}$

By the Yoneda lemma, the n-simplices of a simplicial set X are classified by natural transformations in hom(Δn, X). (Specifically, consider ${\displaystyle \Delta ^{n}=\Delta ^{\mathrm {op} }({\mathbf {n} },-)}$, then the Yoneda lemma gives ${\displaystyle {\mathrm {Nat} }(\Delta ^{\mathrm {op} }({\mathbf {n} },-),X)\cong X({\mathbf {n} })}$) The n-simplices of X are then collectively denoted by Xn. Furthermore, there is a category of simplices, denoted by ${\displaystyle \Delta \downarrow {X}}$ whose objects are maps (i.e. natural transformations) ΔnX and whose morphisms are natural transformations Δn → Δm over X arising from maps [n] [m] in Δ. That is, ${\displaystyle \Delta \downarrow {X}}$ is a slice category of Δ over X. The following isomorphism shows that a simplicial set X is a colimit of its simplices:[2]

${\displaystyle X\cong \varinjlim _{\Delta ^{n}\to X}\Delta ^{n}}$

where the colimit is taken over the category of simplices of X.

## Geometric realization

There is a functor |•|: sSet CGHaus called the geometric realization taking a simplicial set X to its corresponding realization in the category of compactly-generated Hausdorff topological spaces.

This larger category is used as the target of the functor because, in particular, a product of simplicial sets

${\displaystyle X\times Y}$

is realized as a product

${\displaystyle |X|\times _{Ke}|Y|}$

of the corresponding topological spaces, where ${\displaystyle \times _{Ke}}$ denotes the Kelley space product. This product is the right adjoint functor that takes X to XC as described here, applied to the ordinary topological product |X| × |Y|.

To define the realization functor, we first define it on n-simplices Δn as the corresponding topological n-simplex |Δn|. The definition then naturally extends to any simplicial set X by setting

|X| = limΔn → X |Δn|

where the colimit is taken over the n-simplex category of X. The geometric realization is functorial on sSet.

## Singular set for a space

The singular set of a topological space Y is the simplicial set S(Y) defined by

S(Y)([n]) = homTop(|Δn|, Y) for each object [n] ∈ Δ,

with the obvious functoriality condition on the morphisms. This definition is analogous to a standard idea in singular homology of "probing" a target topological space with standard topological n-simplices. Furthermore, the singular functor S is right adjoint to the geometric realization functor described above, i.e.:

homTop(|X|, Y) ≅ homS(X, SY)

for any simplicial set X and any topological space Y.

## Homotopy theory of simplicial sets

In the category of simplicial sets one can define fibrations to be Kan fibrations. A map of simplicial sets is defined to be a weak equivalence if its geometric realization is a weak equivalence of spaces. A map of simplicial sets is defined to be a cofibration if it is a monomorphism of simplicial sets. It is a difficult theorem of Daniel Quillen that the category of simplicial sets with these classes of morphisms satisfies the axioms for a proper closed simplicial model category.

A key turning point of the theory is that the geometric realization of a Kan fibration is a Serre fibration of spaces. With the model structure in place, a homotopy theory of simplicial sets can be developed using standard homotopical algebra methods. Furthermore, the geometric realization and singular functors give a Quillen equivalence of closed model categories inducing an equivalence of homotopy categories

|•|: Ho(sSet) ↔ Ho(Top)

between the homotopy category for simplicial sets and the usual homotopy category of CW complexes with homotopy classes of maps between them. It is part of the general definition of a Quillen adjunction that the right adjoint functor (in this case, the singular set functor) carries fibrations (resp. trivial fibrations) to fibrations (resp. trivial fibrations).

## Simplicial objects

A simplicial object X in a category C is a contravariant functor

X: Δ → C

or equivalently a covariant functor

X: ΔopC

When C is the category of sets, we are just talking about simplicial sets. Letting C be the category of groups or category of abelian groups, we obtain the categories sGrp of simplicial groups and sAb of simplicial abelian groups, respectively.

Simplicial groups and simplicial abelian groups also carry closed model structures induced by that of the underlying simplicial sets.

The homotopy groups of simplicial abelian groups can be computed by making use of the Dold-Kan correspondence which yields an equivalence of categories between simplicial abelian groups and bounded chain complexes and is given by functors

N: sAb → Ch+

and

Γ: Ch+ sAb.

## History and uses of simplicial sets

Simplicial sets were originally used to give precise and convenient descriptions of classifying spaces of groups. This idea was vastly extended by Grothendieck's idea of considering classifying spaces of categories, and in particular by Quillen's work of algebraic K-theory. In this work, which earned him a Fields Medal, Quillen developed surprisingly efficient methods for manipulating infinite simplicial sets. Later these methods were used in other areas on the border between algebraic geometry and topology. For instance, the André-Quillen homology of a ring is a "non-abelian homology", defined and studied in this way.

Both the algebraic K-theory and the André-Quillen homology are defined using algebraic data to write down a simplicial set, and then taking the homotopy groups of this simplicial set. Sometimes one simply defines the algebraic ${\displaystyle K}$-theory as the space.

Simplicial methods are often useful when you want to prove that a space is a loop space. The basic idea is that if ${\displaystyle G}$ is a group with classifying space ${\displaystyle BG}$, then ${\displaystyle G}$ is homotopy equivalent to the loop space ${\displaystyle \Omega BG}$. If ${\displaystyle BG}$ itself is a group, we can iterate the procedure, and ${\displaystyle G}$ is homotopy equivalent to the double loop space ${\displaystyle \Omega ^{2}B(BG)}$. In case ${\displaystyle G}$ is an abelian group, we can actually iterate this infinitely many times, and obtain that ${\displaystyle G}$ is an infinite loop space.

Even if ${\displaystyle X}$ is not an Abelian group, it can happen that it has a composition which is sufficiently commutative so that one can use the above idea to prove that ${\displaystyle X}$ is an infinite loop space. In this way, one can prove that the algebraic ${\displaystyle K}$-theory of a ring, considered as a topological space, is an infinite loop space.

In recent years, simplicial sets have been used in higher category theory and derived algebraic geometry. Quasi-categories can be thought of as categories in which the composition of morphisms is defined only up to homotopy, and information about the composition of higher homotopies is also retained. Quasi-categories are defined as simplicial sets satisfying one additional condition, the weak Kan condition.

## Notes

1. S. Gelfand, Yu. Manin, "Methods of Homological Algebra"