# Kerodon

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Lemma 4.3.6.14 (Joyal [MR1935979]). Let $p,q \geq 0$ be nonnegative integers. Then:

• Assume $p > 0$. Then, for $0 \leq i \leq p$, the pushout-join monomorphism

$(\Lambda ^{p}_{i} \star \Delta ^ q) \coprod _{ ( \Lambda ^{p}_{i} \star \operatorname{\partial \Delta }^ q) } ( \Delta ^ p \star \operatorname{\partial \Delta }^ q) \hookrightarrow \Delta ^{p} \star \Delta ^ q$

of Construction 4.3.6.3 is isomorphic to the horn inclusion $\Lambda ^{p+1+q}_{i} \hookrightarrow \Delta ^{p+1+q}$.

• Assume $q> 0$. Then, for $0 \leq j \leq q$, the pushout-join monomorphism

$(\operatorname{\partial \Delta }^ p \star \Delta ^ q) \coprod _{ ( \operatorname{\partial \Delta }^ p \star \Lambda ^{q}_{j} ) } ( \Delta ^ p \star \Lambda ^ q_ j) \hookrightarrow \Delta ^{p} \star \Delta ^ q$

of Construction 4.3.6.3 is isomorphic to the horn inclusion $\Lambda ^{p+1+q}_{p+1+j} \hookrightarrow \Delta ^{p+1+q}$.

Proof. We will prove the first assertion; the second follows by symmetry. We begin by observing that there is a unique isomorphism of simplicial sets $u: \Delta ^{p} \star \Delta ^{q} \simeq \Delta ^{p+1+q}$ (Example 4.3.3.22). Let $\sigma$ be an $n$-simplex of the join $\Delta ^{p} \star \Delta ^{q}$; we wish to show that $u(\sigma )$ belongs to the horn $\Lambda ^{p+1+q}_{i}$ if and only if $\sigma$ belongs to the union of the simplicial subsets

$\Lambda ^{p}_{i} \star \Delta ^{q} \subseteq \Delta ^{p} \star \Delta ^{q} \supseteq \Delta ^{p} \star \operatorname{\partial \Delta }^{q}.$

We consider three cases (see Remark 4.3.3.15):

• The simplex $\sigma$ belongs to the simplicial subset $\Delta ^{p} \subseteq \Delta ^{p} \star \Delta ^{q}$. In this case, $\sigma$ is contained in $\Delta ^{p} \star \operatorname{\partial \Delta }^{q}$ and $u(\sigma )$ is contained in $\Lambda ^{p+1+q}_{i}$.

• The simplex $\sigma$ belongs to the simplicial subset $\Delta ^{q} \subseteq \Delta ^{p} \star \Delta ^{q}$. In this case, $\sigma$ is contained in $\Lambda ^{p}_{i} \star \Delta ^{q}$ and $u(\sigma )$ is contained in $\Lambda ^{p+1+q}_{i}$ (since $p > 0$).

• The simplex $\sigma$ factors as a composition

$\Delta ^{n} = \Delta ^{p' + 1 + q'} \simeq \Delta ^{p'} \star \Delta ^{q'} \xrightarrow { \sigma _{-} \star \sigma _{+} } \Delta ^{p} \star \Delta {q}.$

Let us abuse notation by identifying $\sigma _{-}$ and $\sigma _{+}$ with nondecreasing functions $[p'] \rightarrow [p]$ and $[q'] \rightarrow [q]$, and $u(\sigma )$ with the nondecreasing function $[n] \rightarrow [p+1+q]$ given by their join. In this case, $\sigma$ fails to belong to the union $(\Lambda ^{p}_{i} \star \Delta ^{q}) \cup ( \Delta ^{p} \star \operatorname{\partial \Delta }^{q} )$ if and only if both of the following conditions are satisfied:

• The image of the nondecreasing function $\sigma _{-}: [p'] \rightarrow [p]$ contains $[p] \setminus \{ i\}$.

• The nondecreasing function $\sigma _{+}: [q'] \rightarrow [q]$ is surjective.

Together, these are equivalent to the assertion that the image of the nondecreasing function $u(\sigma ): [n] \rightarrow [p+1+q]$ contains $[p+1+q] \setminus \{ i\}$: that is, it fails to belong to the horn $\Lambda ^{p+1+q}_{i} \subseteq \Delta ^{p+1+q}$.

$\square$