Lemma 4.41.1 (2-Yoneda lemma for fibred categories). Let \mathcal{C} be a category. Let \mathcal{S} \to \mathcal{C} be a fibred category over \mathcal{C}. Let U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}). The functor
\mathop{\mathrm{Mor}}\nolimits _{\textit{Fib}/\mathcal{C}}(\mathcal{C}/U, \mathcal{S}) \longrightarrow \mathcal{S}_ U
given by G \mapsto G(\text{id}_ U) is an equivalence.
Proof.
Make a choice of pullbacks for \mathcal{S} (see Definition 4.33.6). We define a functor
\mathcal{S}_ U \longrightarrow \mathop{\mathrm{Mor}}\nolimits _{\textit{Fib}/\mathcal{C}}(\mathcal{C}/U, \mathcal{S})
as follows. Given x \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{S}_ U) the associated functor is
on objects: (f : V \to U) \mapsto f^*x, and
on morphisms: the arrow (g : V'/U \to V/U) maps to the composition
(f \circ g)^*x \xrightarrow {(\alpha _{g, f})_ x} g^*f^*x \rightarrow f^*x
where \alpha _{g, f} is as in Lemma 4.33.7.
We omit the verification that this is an inverse to the functor of the lemma.
\square
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