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1.3.6 The Homotopy Category of a Simplicial Set

We now show that the functor $\operatorname{\mathcal{C}}\mapsto \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ of Construction admits a left adjoint (Corollary

Definition Let $\operatorname{\mathcal{C}}$ be a category. We will say that a map of simplicial sets $u: S \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ exhibits $\operatorname{\mathcal{C}}$ as the homotopy category of $S$ if, for every category $\operatorname{\mathcal{D}}$, the composite map

\[ \operatorname{Hom}_{\operatorname{Cat}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{D}}) \rightarrow \operatorname{Hom}_{ \operatorname{Set_{\Delta }}}( \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}), \operatorname{N}_{\bullet }(\operatorname{\mathcal{D}}) ) \xrightarrow {\circ u} \operatorname{Hom}_{\operatorname{Set_{\Delta }}}( S, \operatorname{N}_{\bullet }(\operatorname{\mathcal{D}}) ) \]

is bijective (note that the map on the left is always bijective, by virtue of Proposition

Exercise Let $X$ be a topological space and let $\pi _{\leq 1}(X)$ denote its fundamental groupoid. Show that there is a unique map of simplicial sets $u: \operatorname{Sing}_{\bullet }(X) \rightarrow \operatorname{N}_{\bullet }( \pi _{\leq 1}(X) )$ with the following properties:

  • On $0$-simplices, $u$ carries each point $x \in X$ (regarded as a vertex of $\operatorname{Sing}_{\bullet }(X)$) to itself (regarded as an object of $\pi _{\leq 1}(X)$).

  • On $1$-simplices, $u$ carries each path $p: [0,1] \rightarrow X$ (regarded as an edge of $\operatorname{Sing}_{\bullet }(X)$) to its homotopy class $[p]$ (regarded as a morphism of the category $\pi _{\leq 1}(X)$).

Moreover, $u$ exhibits the fundamental groupoid $\pi _{\leq 1}(X)$ as a homotopy category of the singular simplicial set $\operatorname{Sing}_{\bullet }(X)$. For a generalization, see Proposition

Notation Let $S$ be a simplicial set. It follows immediately from the definition that if there exists a category $\operatorname{\mathcal{C}}$ and a morphism $u: S \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ which exhibits $\operatorname{\mathcal{C}}$ as a homotopy category of $S$, then the category $\operatorname{\mathcal{C}}$ is unique up to isomorphism and depends functorially on $S$. To emphasize this dependence, we will refer to $\operatorname{\mathcal{C}}$ as the homotopy category of $S$ and denote it by $\mathrm{h} \mathit{S}$.

Proposition Let $S = S_{\bullet }$ be a simplicial set. Then there exists a category $\operatorname{\mathcal{C}}$ and a map of simplicial sets $u: S \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ which exhibits $\operatorname{\mathcal{C}}$ as a homotopy category of $S$.

Proof. Let $Q^{\bullet }$ denote the cosimplicial object of $\operatorname{Cat}$ given by the inclusion $\operatorname{{\bf \Delta }}\hookrightarrow \operatorname{Cat}$. Unwinding the definitions, we see that a homotopy category of $S$ can be identified with a realization $| S |^{Q}$, whose existence is a special case of Proposition Alternatively, we can give a direct construction of the homotopy category $\mathrm{h} \mathit{S}$:

  • The objects of $\mathrm{h} \mathit{S}$ are the vertices of $S$.

  • Every edge $e$ of $S$ determines a morphism $[e]$ in $\mathrm{h} \mathit{S}$, whose source is the vertex $d^{1}_1(e)$ and whose target is the vertex $d^{1}_0(e)$.

  • The collection of morphisms in $\mathrm{h} \mathit{S}$ is generated under composition by morphisms of the form $[e]$, subject only to the relations

    \[ [ s^{0}_0(x) ] = \operatorname{id}_ x \text{ for $x \in S_0$ } \quad \quad [ d^{2}_1(\sigma ) ] = [ d^{2}_0(\sigma ) ] \circ [ d^{2}_2(\sigma ) ] \text{ for $\sigma \in S_2$. } \]

Corollary The nerve functor $\operatorname{N}_{\bullet }: \operatorname{Cat}\rightarrow \operatorname{Set_{\Delta }}$ admits a left adjoint, given on objects by the construction $S \mapsto \mathrm{h} \mathit{S}$.

Remark Let $\operatorname{\mathcal{C}}$ be a category. Then the counit of the adjunction described in Corollary induces an isomorphism of categories $\mathrm{h} \mathit{\operatorname{N}}_{\bullet }(\operatorname{\mathcal{C}}) \xrightarrow {\sim } \operatorname{\mathcal{C}}$ (this is a restatement of Proposition In other words, every category $\operatorname{\mathcal{C}}$ can be recovered as the homotopy category of its nerve $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$.

Warning Let $S$ be a simplicial set. The proof of Proposition gives a construction of the homotopy category $\mathrm{h} \mathit{S}$ by generators and relations. The result of this construction is not always easy to describe. If $x$ and $y$ are vertices of $S$, then every morphism from $x$ to $y$ in $\mathrm{h} \mathit{S}$ can be represented by a composition

\[ [e_ n] \circ [e_{n-1}] \circ \cdots \circ [e_1], \]

where $\{ e_ i \} _{0 \leq i \leq n}$ is a sequence of edges satisfying

\[ d^{1}_1(e_1) = x \quad \quad d^{1}_0(e_ i) = d^{1}_1(e_{i+1}) \quad \quad d^{1}_0( e_ n ) = y. \]

In general, it can be difficult to determine whether or not two such compositions represent the same morphism of $\mathrm{h} \mathit{S}$ (even for finite simplicial sets, this question is algorithmically undecidable). However, there are two situations in which the homotopy category $\mathrm{h} \mathit{S}$ admits a simpler description:

  • Let $S$ be a simplicial set of dimension $\leq 1$, which we can identify with a directed graph $G$ (Proposition In this case, the homotopy category $\mathrm{h} \mathit{S}$ is generated freely by the vertices and edges of the graph $G$: that is, it can be identified with the path category of $G$ (Proposition which we study in §1.3.7.

  • Let $S$ be an $\infty $-category. In this case, every morphism in the homotopy category $\operatorname{\mathcal{C}}= \mathrm{h} \mathit{S}$ can be represented by a single edge of $S$, rather than a composition of edges (in other words, the canonical map $u: S \rightarrow \operatorname{N}_{\bullet }( \operatorname{\mathcal{C}})$ is surjective on edges), and two edges of $S$ represent the same morphism in $\mathrm{h} \mathit{S}$ if and only if they are homotopic (Definition This leads to a more explicit description of the homotopy category $\operatorname{\mathcal{C}}$ (generalizing Exercise which we will discuss in §1.4.5 (see Proposition