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Aug 7 at 9:17 comment added Achim Krause AH, I think I know what's going on: When they write $\mathrm{const}_1(\mathcal{C})$, they don't mean your $\mathrm{const}_1$, instead they mean the simplicial diagram left Kan extended from the functor $\Delta_{\leq 1}\to \mathrm{Cat}$ taking $[0]\mapsto 0$ and $[1]\mapsto \mathcal{C}$. This should realize to the suspension of $\mathcal{C}^\simeq$, and a map $\Sigma\mathcal{C}^\simeq \to |Q(\mathcal{C})|$ corresponds to a map $\mathcal{C}^\simeq \to \Omega|Q(\mathcal{C})|$ by adjunction.
Aug 7 at 1:49 vote accept XiaYu
Aug 7 at 1:39 comment added XiaYu And I totally agree with you that this shouldn't be a equivalence.
Aug 7 at 1:35 comment added XiaYu Thanks for your answer. In 4.1.27 $const_1(C)\rightarrow Q(C)$ only implies $|const_1(C^{\simeq})|\rightarrow |Q(C)^{\simeq}|$, but $K(C)=\Omega |Q(C)^{\simeq}|$(as in definition 4.1.24) so it seems that there may be some natural morphism $C^{\simeq}\rightarrow \Omega|const_1(C^{\simeq})|$. That's why I expected something about $\Omega|const_1(C^{\simeq})|$ to be told.
Aug 6 at 13:23 history answered Achim Krause CC BY-SA 4.0