Lemma 33.15.5. Let k be a field. Let X be a scheme over k. If X_ K is projective over K for some field extension K/k, then X is projective over k.
Proof. A scheme over k is projective over k if and only if it is quasi-projective and proper over k. See Morphisms, Lemma 29.43.13. Thus the lemma follows from Lemmas 33.15.3 and 33.15.4. \square
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