Definition 35.2.3. Let $S$ be a scheme. Let $\{ S_ i \to S\} _{i \in I}$ be a family of morphisms with target $S$.

Let $\mathcal{F}$ be a quasi-coherent $\mathcal{O}_ S$-module. We call the unique descent on $\mathcal{F}$ datum with respect to the covering $\{ S \to S\} $ the

*trivial descent datum*.The pullback of the trivial descent datum to $\{ S_ i \to S\} $ is called the

*canonical descent datum*. Notation: $(\mathcal{F}|_{S_ i}, can)$.A descent datum $(\mathcal{F}_ i, \varphi _{ij})$ for quasi-coherent sheaves with respect to the given covering is said to be

*effective*if there exists a quasi-coherent sheaf $\mathcal{F}$ on $S$ such that $(\mathcal{F}_ i, \varphi _{ij})$ is isomorphic to $(\mathcal{F}|_{S_ i}, can)$.

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