Definition 59.16.1. Let $\mathcal{U} = \{ t_ i : T_ i \to T\} _{i \in I}$ be a family of morphisms of schemes with fixed target. A *descent datum* for quasi-coherent sheaves with respect to $\mathcal{U}$ is a collection $((\mathcal{F}_ i)_{i \in I}, (\varphi _{ij})_{i, j \in I})$ where

$\mathcal{F}_ i$ is a quasi-coherent sheaf on $T_ i$, and

$\varphi _{ij} : \text{pr}_0^* \mathcal{F}_ i \to \text{pr}_1^* \mathcal{F}_ j$ is an isomorphism of modules on $T_ i \times _ T T_ j$,

such that the *cocycle condition* holds: the diagrams

commute on $T_ i \times _ T T_ j \times _ T T_ k$. This descent datum is called *effective* if there exist a quasi-coherent sheaf $\mathcal{F}$ over $T$ and $\mathcal{O}_{T_ i}$-module isomorphisms $\varphi _ i : t_ i^* \mathcal{F} \cong \mathcal{F}_ i$ compatible with the maps $\varphi _{ij}$, namely

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