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1.3.2 The Opposite of an $\infty$-Category

Let $\operatorname{\mathcal{C}}$ be an ordinary category. Then we can construct a new category $\operatorname{\mathcal{C}}^{\operatorname{op}}$, called the opposite category of $\operatorname{\mathcal{C}}$, as follows:

• The objects of the opposite category $\operatorname{\mathcal{C}}^{\operatorname{op}}$ are the objects of $\operatorname{\mathcal{C}}$.

• For every pair of objects $C,D \in \operatorname{\mathcal{C}}$, we have $\operatorname{Hom}_{ \operatorname{\mathcal{C}}^{\operatorname{op}} }( C, D) = \operatorname{Hom}_{\operatorname{\mathcal{C}}}( D, C )$.

• Composition of morphisms in $\operatorname{\mathcal{C}}^{\operatorname{op}}$ is given by the composition of morphisms in $\operatorname{\mathcal{C}}$, with the order reversed.

The construction $\operatorname{\mathcal{C}}\mapsto \operatorname{\mathcal{C}}^{\operatorname{op}}$ admits a straightforward generalization to the setting of $\infty$-categories. In fact, it can be extended to arbitrary simplicial sets.

Notation 1.3.2.1. Let $\operatorname{Lin}$ denote the category whose objects are finite linearly ordered sets and whose morphisms are nondecreasing functions. Let $I$ be an object of $\operatorname{Lin}$, regarded as a set with a linear ordering $\leq _{I}$. We let $I^{\operatorname{op}}$ denote the same set with the opposite ordering, so that

$( i \leq _{I^{\operatorname{op}} } j ) \Leftrightarrow (j \leq _{I} i ).$

The construction $I \mapsto I^{\operatorname{op}}$ determines an equivalence from the category $\operatorname{Lin}$ to itself.

Recall that the simplex category $\operatorname{{\bf \Delta }}$ of Definition 1.1.1.2 is the full subcategory of $\operatorname{Lin}$ spanned by objects of the form $[n] = \{ 0 < 1 < \cdots < n \}$, and is equivalent to the full subcategory of $\operatorname{Lin}$ spanned by those linearly ordered sets which are finite and nonempty (Remark 1.1.1.3). There is a unique functor $\mathrm{Op}: \operatorname{{\bf \Delta }}\rightarrow \operatorname{{\bf \Delta }}$ for which the diagram

$\xymatrix { \operatorname{{\bf \Delta }}\ar [r] \ar [d]^{\mathrm{Op} } & \operatorname{Lin}\ar [d]^{I \mapsto I^{\operatorname{op}} } \\ \operatorname{{\bf \Delta }}\ar [r] & \operatorname{Lin}}$

commutes up to isomorphism, where the horizontal maps are given by the inclusion. The functor $\mathrm{Op}$ can be described more concretely as follows:

• For each object $[n] \in \operatorname{{\bf \Delta }}$, we have $\mathrm{Op}([n]) = [n]$ (note that the construction $i \mapsto n-i$ determines an isomorphism of $[n]$ with the opposite linear ordering $[n]^{\operatorname{op}}$).

• For each morphism $\alpha : [m] \rightarrow [n]$ in $\operatorname{{\bf \Delta }}$, the morphism $\mathrm{Op}( \alpha ): [m] \rightarrow [n]$ is given by the formula $\mathrm{Op}( \alpha )(i) = n - \alpha (m-i)$.

Construction 1.3.2.2. Let $S_{\bullet }$ be a simplicial set, which we regard as a functor $\operatorname{{\bf \Delta }}^{\operatorname{op}} \rightarrow \operatorname{Set}$. We let $S_{\bullet }^{\operatorname{op}}$ denote the simplicial set given by the composition

$\operatorname{{\bf \Delta }}^{\operatorname{op}} \xrightarrow { \mathrm{Op} } \operatorname{{\bf \Delta }}^{\operatorname{op}} \xrightarrow { S_{\bullet } } \operatorname{Set},$

where $\mathrm{Op}$ is the functor described in Notation 1.3.2.1. We will refer to $S_{\bullet }^{\operatorname{op}}$ as the opposite of the simplicial set $S_{\bullet }$.

Remark 1.3.2.3. Let $S_{\bullet }$ be a simplicial set. Then the opposite simplicial set $S_{\bullet }^{\operatorname{op}}$ can be described more concretely as follows:

• For each $n \geq 0$, we have $S_{n}^{\operatorname{op}} = S_{n}$.

• The face and degeneracy maps of $S_{\bullet }^{\operatorname{op}}$ are given by

$(d_ i: S^{\operatorname{op}}_ n \rightarrow S^{\operatorname{op}}_{n-1}) = (d_{n-i}: S_ n \rightarrow S_{n-1})$
$(s_ i: S^{\operatorname{op}}_ n \rightarrow S^{\operatorname{op}}_{n+1}) = (s_{n-i}: S_ n \rightarrow S_{n+1}).$

Example 1.3.2.4. Let $\operatorname{\mathcal{C}}$ be a category. For each $n \geq 0$, we can identify $n$-simplices $\sigma$ of $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ with diagrams

$C_0 \xrightarrow {f_1} C_1 \xrightarrow {f_2} \cdots \xrightarrow {f_{n-1}} C_{n-1} \xrightarrow {f_ n} C_ n$

in the category $\operatorname{\mathcal{C}}$. Then $\sigma$ determines an $n$-simplex $\sigma '$ of $\operatorname{N}_{\bullet }( \operatorname{\mathcal{C}}^{\operatorname{op}} )$, given by the diagram

$C_ n \xrightarrow {f_ n} C_{n-1} \xrightarrow { f_{n-1} } \cdots \xrightarrow {f_2} C_1 \xrightarrow {f_1} C_0$

in the opposite category $\operatorname{\mathcal{C}}^{\operatorname{op}}$. The construction $\sigma \mapsto \sigma '$ determines an isomorphism of simplicial sets $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})^{\operatorname{op}} \simeq \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}^{\operatorname{op}} )$.

Example 1.3.2.5. Let $X$ be a topological space. Then there is a canonical isomorphism of simplicial sets $\operatorname{Sing}_{\bullet }(X) \simeq \operatorname{Sing}_{\bullet }(X)^{\operatorname{op}}$, which carries each singular $n$-simplex $\sigma : | \Delta ^{n} | \rightarrow X$ to the composite map

$| \Delta ^{n} | \xrightarrow {r} | \Delta ^ n | \xrightarrow {\sigma } X$

where $r$ is denotes the homeomorphism of $| \Delta ^ n |$ with itself given by $r( t_0, t_1, \ldots , t_{n-1}, t_ n) = (t_ n, t_{n-1}, \ldots , t_1, t_0)$.

Proposition 1.3.2.6. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. Then the opposite simplicial set $\operatorname{\mathcal{C}}^{\operatorname{op}}$ is also an $\infty$-category.

Proof. Let $\sigma _0: \Lambda ^{n}_{i} \rightarrow \operatorname{\mathcal{C}}^{\operatorname{op}}$ be a map of simplicial sets for $0 < i < n$; we wish to show that $\sigma _0$ can be extended to an $n$-simplex of $\operatorname{\mathcal{C}}^{\operatorname{op}}$. Passing to opposite simplicial sets, we are reduced to showing that the map $\sigma _0^{\operatorname{op}}: ( \Lambda ^{n}_{i})^{\operatorname{op}} \rightarrow \operatorname{\mathcal{C}}$ can be extended to a map $( \Delta ^ n )^{\operatorname{op}} \rightarrow \operatorname{\mathcal{C}}$. This follows from our assumption that $\operatorname{\mathcal{C}}$ is an $\infty$-category, since there is a canonical isomorphism $( \Delta ^ n)^{\operatorname{op}} \simeq \Delta ^ n$ which carries the simplicial subset $(\Lambda ^{n}_{i})^{\operatorname{op}}$ to $\Lambda ^{n}_{n-i}$. $\square$

Remark 1.3.2.7. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. We will refer to the $\infty$-category $\operatorname{\mathcal{C}}^{\operatorname{op}}$ of Proposition 1.3.2.6 as the opposite of the $\infty$-category $\operatorname{\mathcal{C}}$. Note that:

• The objects of $\operatorname{\mathcal{C}}^{\operatorname{op}}$ are the objects of $\operatorname{\mathcal{C}}$.

• Given a pair of objects $X,Y \in \operatorname{\mathcal{C}}$, the datum of a morphism from $X$ to $Y$ in $\operatorname{\mathcal{C}}^{\operatorname{op}}$ is equivalent to the datum of a morphism from $Y$ to $X$ in $\operatorname{\mathcal{C}}$.