Definition 44.4.1. Let \mathit{Sch}_{fppf} be a big site as in Topologies, Definition 34.7.8. Let f : X \to S be a morphism of this site. The Picard functor \mathrm{Pic}_{X/S} is the fppf sheafification of the functor
(\mathit{Sch}/S)_{fppf} \longrightarrow \textit{Sets},\quad T \longmapsto \mathop{\mathrm{Pic}}\nolimits (X_ T)
If this functor is representable, then we denote \underline{\mathrm{Pic}}_{X/S} a scheme representing it.
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