Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$

Construction 5.1.1.12 (The Contravariant Transport Functor). Let $q: X \rightarrow S$ be a right fibration of simplicial sets, and let $\mathrm{h} \mathit{\operatorname{Kan}}$ denote the homotopy category of Kan complexes (Construction 3.1.4.10). Dualizing Construction 5.1.1.7, we deduce that there is a unique map of simplicial sets $T: S^{\operatorname{op}} \rightarrow \operatorname{N}_{\bullet }( \mathrm{h} \mathit{\operatorname{Kan}} )$ with the following properties:

  • For each vertex $s \in S$, we have $T(s) = X_{s}$.

  • For each edge $e: s \rightarrow s'$ of the simplicial set $S$, we have $T(e) = [ e^{\ast } ]$. Here $e^{\ast }: X_{s'} \rightarrow X_{s}$ denotes a morphism of Kan complexes given by contravariant transport along $e$, and $[e^{\ast }]$ denotes its homotopy class.

The morphism $T$ determines a functor from the homotopy category $\mathrm{h} \mathit{S}^{\operatorname{op}}$ (Notation 1.2.5.3) to the homotopy category $\mathrm{h} \mathit{\operatorname{Kan}}$, which we will also denote by $T: \mathrm{h} \mathit{S}^{\operatorname{op}} \rightarrow \mathrm{h} \mathit{\operatorname{Kan}}$. We will refer to $T$ as the contravariant transport functor.