# The Stacks Project

## Tag 03N0

Lemma 34.10.2. Let $\{f_i : T_i \to T\}_{i \in I}$ be a fpqc covering. Suppose that for each $i$ we have an open subset $W_i \subset T_i$ such that for all $i, j \in I$ we have $\text{pr}_0^{-1}(W_i) = \text{pr}_1^{-1}(W_j)$ as open subsets of $T_i \times_T T_j$. Then there exists a unique open subset $W \subset T$ such that $W_i = f_i^{-1}(W)$ for each $i$.

Proof. Apply Lemma 34.10.1 to the map $\coprod_{i \in I} T_i \to T$. It implies there exists a subset $W \subset T$ such that $W_i = f_i^{-1}(W)$ for each $i$, namely $W = \bigcup f_i(W_i)$. To see that $W$ is open we may work Zariski locally on $T$. Hence we may assume that $T$ is affine. Using the definition of a fpqc covering, this reduces us to the case where $\{f_i : T_i \to T\}$ is a standard fpqc covering. In this case we may apply Morphisms, Lemma 28.24.11 to the morphism $\coprod T_i \to T$ to conclude that $W$ is open. $\square$

The code snippet corresponding to this tag is a part of the file descent.tex and is located in lines 3228–3236 (see updates for more information).

\begin{lemma}
\label{lemma-open-fpqc-covering}
Let $\{f_i : T_i \to T\}_{i \in I}$ be a fpqc covering.
Suppose that for each $i$ we have an open subset $W_i \subset T_i$
such that for all $i, j \in I$ we have
$\text{pr}_0^{-1}(W_i) = \text{pr}_1^{-1}(W_j)$ as open
subsets of $T_i \times_T T_j$. Then there exists a unique open subset
$W \subset T$ such that $W_i = f_i^{-1}(W)$ for each $i$.
\end{lemma}

\begin{proof}
Apply
Lemma \ref{lemma-equiv-fibre-product}
to the map $\coprod_{i \in I} T_i \to T$.
It implies there exists a subset $W \subset T$ such that
$W_i = f_i^{-1}(W)$ for each $i$, namely $W = \bigcup f_i(W_i)$.
To see that $W$ is open we may work Zariski locally on $T$.
Hence we may assume that $T$ is affine. Using the definition
of a fpqc covering, this reduces us to the case where
$\{f_i : T_i \to T\}$ is a standard fpqc covering. In this case we
may apply
Morphisms, Lemma \ref{morphisms-lemma-fpqc-quotient-topology}
to the morphism
$\coprod T_i \to T$ to conclude that $W$ is open.
\end{proof}

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