Kerodon

Proof. Combine Proposition 4.5.2.15 with Corollary 4.7.8.2. $\square$

Proof. For $n < 0$, the result is trivial (see Example 4.7.7.10). We will therefore assume that $n \geq 0$. It follows from Corollary 4.7.7.23 that the morphism space $\operatorname{Hom}_{\operatorname{\mathcal{C}}}(X,Y) = \{ X\} \vec{\times }_{\operatorname{\mathcal{C}}} \{ Y\} $ is also $n$-category; in particular, it is minimal in dimensions $\geq n$. Since $\operatorname{Hom}_{\operatorname{\mathcal{C}}}(X,Y)$ is $(n-1)$-truncated (Warning 4.7.7.21), it is locally $(n-2)$-truncated (Example 4.7.6.10). Applying Proposition 4.7.8.1, we conclude that $\operatorname{Hom}_{\operatorname{\mathcal{C}}}(X,Y)$ is weakly $(n-1)$-coskeletal. $\square$

Proof of Proposition 4.7.8.6. Let $\operatorname{\mathcal{C}}$ be an $\infty $-category and let $m > 0$ be an integer. It follows immediately from the definitions that, if the restriction map

is injective, then $\operatorname{\mathcal{C}}$ is minimal in dimension $m$. We claim that, if this condition is satisfied, then $\theta _{m+1}$ is surjective: that is, every morphism $\tau _0: \operatorname{\partial \Delta }^{m+1} \rightarrow \operatorname{\mathcal{C}}$ can be extended to an $(m+1)$-simplex of $\operatorname{\mathcal{C}}$. Fix an integer $0 < i < m+1$. Our assumption that $\operatorname{\mathcal{C}}$ is an $\infty $-category guarantees that we can choose an $(m+1)$-simplex $\tau $ of $\operatorname{\mathcal{C}}$ satisfying $\tau |_{ \Lambda ^{m+1}_{i} } = \tau _0 |_{ \Lambda ^{m+1}_{i} }$. In particular, $\tau $ and $\tau _0$ have the same restriction to the $(m-1)$-skeleton of $\Delta ^{m+1}$. Invoking the injectivity of $\theta _{m}$, we conclude that $\tau |_{ \operatorname{\partial \Delta }^{m+1} } = \tau _0$.

We now prove the converse. Assume that $\operatorname{\mathcal{C}}$ is minimal in dimension $m$ and that $\theta _{m+1}$ is surjective; we wish to show that $\theta _{m}$ is injective. Let $\sigma _0$ and $\sigma _1$ be $m$-simplices of $\operatorname{\mathcal{C}}$ which have the same restriction to $\operatorname{\partial \Delta }^{m}$; we wish to show that $\sigma _0 = \sigma _1$. Let

be the filtration of Lemma 3.1.2.12, so that $X(0) = (\Delta ^1 \times \operatorname{\partial \Delta }^ m) \cup ( \{ 1\} \times \Delta ^ m)$ and the inclusion map $X(i) \hookrightarrow X(i+1)$ is inner anodyne for $0 \leq i < m$. There is a unique morphism of simplicial sets $h_0: X(0) \rightarrow \operatorname{\mathcal{C}}$ such that $h_0 |_{ \{ 1\} \times \Delta ^ m }$ coincides with $\sigma _1$, and $h_0|_{ \Delta ^1 \times \operatorname{\partial \Delta }^{m} }$ factors through the projection map $\Delta ^1 \times \operatorname{\partial \Delta }^{m} \twoheadrightarrow \operatorname{\partial \Delta }^ m$. Since $\operatorname{\mathcal{C}}$ is an $\infty $-category, we can extend $h_0$ to a diagram $h_{m}: X(m) \rightarrow \operatorname{\mathcal{C}}$. Invoking the surjectivity of $\theta _{m+1}$, we see that $h_{m}$ can be extended to a morphism $h: \Delta ^1 \times \Delta ^ m \rightarrow \operatorname{\mathcal{C}}$ satisfying $h|_{ \{ 0\} \times \Delta ^ m } = \sigma _0$. By construction, $h$ is an isomorphism from $\sigma _0$ to $\sigma _1$ in the $\infty $-category $\operatorname{Fun}( \Delta ^ m, \operatorname{\mathcal{C}})$ whose image in $\operatorname{Fun}( \operatorname{\partial \Delta }^ m, \operatorname{\mathcal{C}})$ is an identity morphism. Since $\operatorname{\mathcal{C}}$ is minimal in dimension $m$, it follows that $\sigma _0 = \sigma _1$. $\square$

Proof of Proposition 4.7.8.1. Let $\operatorname{\mathcal{C}}$ be an $\infty $-category and let $n$ be an integer. For every integer $m \geq 0$, we let $\theta _{m}: \operatorname{Hom}_{\operatorname{Set_{\Delta }}}( \Delta ^{m}, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Hom}_{\operatorname{Set_{\Delta }}}( \operatorname{\partial \Delta }^{m}, \operatorname{\mathcal{C}})$ denote the restriction map. By virtue of Remark 4.7.6.11, the map $\theta _{m}$ is surjective for $m \geq n+2$ if and only if $\operatorname{\mathcal{C}}$ is locally $(n-1)$-truncated (and nonempty if $n \leq -2$). Assume that these equivalent conditions are satisfied. Then:

$(1)$

The $\infty $-category $\operatorname{\mathcal{C}}$ is $(n+1)$-coskeletal if and only if $\theta _{m}$ is injective for $m \geq n+2$. By virtue of Proposition 4.7.8.6 (and Remark 4.7.8.7), this is equivalent to the requirement that $\operatorname{\mathcal{C}}$ is minimal in dimensions $\geq n+2$.

$(2)$

The $\infty $-category $\operatorname{\mathcal{C}}$ is weakly $n$-coskeletal if and only if $\theta _{m}$ is injective for $m \geq n+1$. By virtue of Proposition 4.7.8.6 (and Remark 4.7.8.7), this is equivalent to the requirement that $\operatorname{\mathcal{C}}$ is minimal in dimensions $\geq n+1$.

$(3)$

The $\infty $-category $\operatorname{\mathcal{C}}$ is an $n$-category if and only if $\operatorname{\mathcal{C}}$ is minimal in dimensions $\geq n$. This follows immediately from $(2)$ (see Definition 4.7.7.9).

Proof. Without loss of generality, we may assume that $X$ is nonempty (otherwise, neither $(a)$ nor $(b)$ is satisfied). In this case, $X$ is $n$-reduced if and only if the restriction map $\theta _{m}: \operatorname{Hom}_{\operatorname{Set_{\Delta }}}( \Delta ^{m}, X) \rightarrow \operatorname{Hom}_{\operatorname{Set_{\Delta }}}( \operatorname{\partial \Delta }^{m}, X)$ is injective for each $m \leq n$. Corollary 4.7.8.8 now follows by combining Proposition 4.7.8.6 (and Remark 4.7.8.7) with the criterion of Proposition 3.5.1.12. $\square$

Proof. For $n < 0$, the weak coskeleton $\operatorname{cosk}_{n}^{\circ }(\operatorname{\mathcal{C}})$ is either empty (if $n = -1$ and $\operatorname{\mathcal{C}}$ is empty) or isomorphic to $\Delta ^0$; in either case, assertions $(1)$ through $(4)$ are clear. We may therefore assume that $n \geq 0$. The map $F$ factors as a composition

where $F''$ is a trivial Kan fibration (Proposition 3.5.4.22). Since $\operatorname{cosk}_{n+1}(\operatorname{\mathcal{C}})$ is an $\infty $-category (Proposition 4.7.6.19), assertion $(1)$ follows from Proposition 1.5.5.11.

To prove $(2)$, we proceed as in Proposition 3.5.4.26. Suppose we are given a pair of integers $0 < i < m$; we wish to show that every lifting problem

admits a solution. We consider two cases:

• If $m \leq n+1$, then we can choose an $m$-simplex $\sigma $ of $\operatorname{\mathcal{C}}$ satisfying $F(\sigma ) = \overline{\sigma }$. Since $F$ is bijective on simplices of dimension $\leq n$, the commutativity of the diagram (4.77) guarantees that $\sigma |_{ \Lambda ^{m}_{i} } = \sigma _0$.

• If $m \geq n+2$, then our assumption that $\operatorname{\mathcal{C}}$ is an $\infty $-category guarantees that $\sigma _0$ can be extended to an $m$-simplex $\sigma $ of $X$. The commutativity of the diagram (4.77) then guarantees that $F(\sigma )$ and $\overline{\sigma }$ have the same restriction to the horn $\Lambda ^{m}_{i} \subset \Delta ^{m}$. In particular, they have the same restriction to the $n$-skeleton of $\Delta ^{m}$, so $F(\sigma ) = \overline{\sigma }$.

Since $F''$ is an equivalence of $\infty $-categories, assertion $(3)$ follows from Proposition 4.7.6.19. It remains to prove $(4)$. Let $Y$ be an object of $\operatorname{\mathcal{C}}$ having image $\overline{Y} \in \operatorname{cosk}^{\circ }_{n}(\operatorname{\mathcal{C}})$ and suppose we are given an isomorphism $\overline{u}: \overline{X} \rightarrow \overline{Y}$ in the $\infty $-category $\operatorname{cosk}_{n}^{\circ }(\operatorname{\mathcal{C}})$. If $n \geq 1$, then $F$ is bijective on vertices and edges; it follows that we can write $\overline{u} = F(u)$ for a unique morphism $u: X \rightarrow Y$ in $\operatorname{\mathcal{C}}$. To complete the proof, it will suffice to show that $u$ is an isomorphism. Equivalently, we wish to show that the homotopy class $[u]$ is an isomorphism in the homotopy category $\operatorname {h}\! \mathit{\operatorname{\mathcal{C}}}$. This is clear: the nerve $\operatorname{N}_{\bullet }( \operatorname {h}\! \mathit{\operatorname{\mathcal{C}}} )$ is weakly $1$-coskeletal (Example 3.5.4.6), so the tautological map $\operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }( \operatorname {h}\! \mathit{\operatorname{\mathcal{C}}} )$ factors (uniquely) through $\operatorname{cosk}^{\circ }_{n}(\operatorname{\mathcal{C}})$ (Proposition 3.5.4.18). $\square$