Lemma 18.27.1. If \mathcal{C} is a site, \mathcal{O} is a sheaf of rings, \mathcal{F} is a presheaf of \mathcal{O}-modules, and \mathcal{G} is a sheaf of \mathcal{O}-modules, then \mathop{\mathcal{H}\! \mathit{om}}\nolimits _\mathcal {O}(\mathcal{F}, \mathcal{G}) is a sheaf of \mathcal{O}-modules.
Proof. Omitted. Hints: Note first that \mathop{\mathcal{H}\! \mathit{om}}\nolimits _\mathcal {O}(\mathcal{F}, \mathcal{G}) = \mathop{\mathcal{H}\! \mathit{om}}\nolimits _\mathcal {O}(\mathcal{F}^\# , \mathcal{G}), which reduces the question to the case where both \mathcal{F} and \mathcal{G} are sheaves. The result for sheaves of sets is Sites, Lemma 7.26.1. \square
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