Lemma 7.5.6. Let $u : \mathcal{C} \to \mathcal{D}$ be a functor between categories. For any object $U$ of $\mathcal{C}$ we have $u_ ph_ U = h_{u(U)}$.

Proof. By adjointness of $u_ p$ and $u^ p$ we have

$\mathop{Mor}\nolimits _{\textit{PSh}(\mathcal{D})}(u_ ph_ U, \mathcal{G}) = \mathop{Mor}\nolimits _{\textit{PSh}(\mathcal{C})}(h_ U, u^ p\mathcal{G}) = u^ p\mathcal{G}(U) = \mathcal{G}(u(U))$

and hence by Yoneda's lemma we see that $u_ ph_ U = h_{u(U)}$ as presheaves. $\square$

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