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The Stacks project

Situation 77.7.1. Let S be a scheme. Let f : X \to B be a morphism of algebraic spaces over S. Let u : \mathcal{F} \to \mathcal{G} be a homomorphism of quasi-coherent \mathcal{O}_ X-modules. For any scheme T over B we will denote u_ T : \mathcal{F}_ T \to \mathcal{G}_ T the base change of u to T, in other words, u_ T is the pullback of u via the projection morphism X_ T = X \times _ B T \to X. In this situation we can consider the functor

77.7.1.1
\begin{equation} \label{spaces-flat-equation-iso} F_{iso} : (\mathit{Sch}/B)^{opp} \longrightarrow \textit{Sets}, \quad T \longrightarrow \left\{ \begin{matrix} \{ *\} & \text{if }u_ T\text{ is an isomorphism}, \\ \emptyset & \text{else.} \end{matrix} \right. \end{equation}

There are variants F_{inj}, F_{surj}, F_{zero} where we ask that u_ T is injective, surjective, or zero.


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