Lemma 100.9.17. Let \mathcal X be an algebraic stack. Let \mathcal{X}_ i, i \in I be a set of open substacks of \mathcal{X}. Assume
\mathcal{X} = \bigcup _{i \in I} \mathcal{X}_ i, and
each \mathcal{X}_ i is a scheme
Then \mathcal{X} is a scheme.
Lemma 100.9.17. Let \mathcal X be an algebraic stack. Let \mathcal{X}_ i, i \in I be a set of open substacks of \mathcal{X}. Assume
\mathcal{X} = \bigcup _{i \in I} \mathcal{X}_ i, and
each \mathcal{X}_ i is a scheme
Then \mathcal{X} is a scheme.
Proof. By Lemma 100.9.16 we see that \mathcal{X} is an algebraic space. Since any algebraic space has a largest open subspace which is a scheme, see Properties of Spaces, Lemma 66.13.1 we see that \mathcal{X} is a scheme. \square
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