Lemma 30.17.6. Let i : Z \to X be a closed immersion of Noetherian schemes inducing a homeomorphism of underlying topological spaces. Then X is quasi-affine if and only if Z is quasi-affine.
Proof. Recall that a scheme is quasi-affine if and only if the structure sheaf is ample, see Properties, Lemma 28.27.1. Hence if Z is quasi-affine, then \mathcal{O}_ Z is ample, hence \mathcal{O}_ X is ample by Lemma 30.17.5, hence X is quasi-affine. A proof of the converse, which can also be seen in an elementary way, is gotten by reading the argument just given backwards. \square
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