Lemma 6.26.2. Let $(f, f^\sharp ) : (X, \mathcal{O}_ X) \to (Y, \mathcal{O}_ Y)$ be a morphism of ringed spaces. Let $\mathcal{F}$ be a sheaf of $\mathcal{O}_ X$-modules. Let $\mathcal{G}$ be a sheaf of $\mathcal{O}_ Y$-modules. There is a canonical bijection

$\mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_ X}(f^*\mathcal{G}, \mathcal{F}) = \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_ Y}(\mathcal{G}, f_*\mathcal{F}).$

In other words: the functor $f^*$ is the left adjoint to $f_*$.

Proof. This follows from the work we did before:

\begin{eqnarray*} \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_ X}(f^*\mathcal{G}, \mathcal{F}) & = & \mathop{Mor}\nolimits _{\textit{Mod}(\mathcal{O}_ X)}( \mathcal{O}_ X \otimes _{f^{-1}\mathcal{O}_ Y} f^{-1}\mathcal{G}, \mathcal{F}) \\ & = & \mathop{Mor}\nolimits _{\textit{Mod}(f^{-1}\mathcal{O}_ Y)}( f^{-1}\mathcal{G}, \mathcal{F}_{f^{-1}\mathcal{O}_ Y}) \\ & = & \mathop{\mathrm{Hom}}\nolimits _{\mathcal{O}_ Y}(\mathcal{G}, f_*\mathcal{F}). \end{eqnarray*}

Here we use Lemmas 6.20.2 and 6.24.7. $\square$

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