Lemma 80.5.2. Let S be a scheme. Let
\xymatrix{ E \ar[r]_ a \ar[d]_ f & F \ar[d]^ g \\ H \ar[r]^ b & G }
be a cartesian diagram of sheaves on (\mathit{Sch}/S)_{fppf}, so E = H \times _ G F. If
g is representable by algebraic spaces, surjective, flat, and locally of finite presentation, and
a is representable by algebraic spaces, separated, and locally quasi-finite
then b is representable (by schemes) as well as separated and locally quasi-finite.
Proof.
Let T be a scheme, and let T \to G be a morphism. We have to show that T \times _ G H is a scheme, and that the morphism T \times _ G H \to T is separated and locally quasi-finite. Thus we may base change the whole diagram to T and assume that G is a scheme. In this case F is an algebraic space. Let U be a scheme, and let U \to F be a surjective étale morphism. Then U \to F is representable, surjective, flat and locally of finite presentation by Morphisms of Spaces, Lemmas 67.39.7 and 67.39.8. By Lemma 80.3.8 U \to G is surjective, flat and locally of finite presentation also. Note that the base change E \times _ F U \to U of a is still separated and locally quasi-finite (by Lemma 80.4.2). Hence we may replace the upper part of the diagram of the lemma by E \times _ F U \to U. In other words, we may assume that F \to G is a surjective, flat morphism of schemes which is locally of finite presentation. In particular, \{ F \to G\} is an fppf covering of schemes. By Morphisms of Spaces, Proposition 67.50.2 we conclude that E is a scheme also. By Descent, Lemma 35.39.1 the fact that E = H \times _ G F means that we get a descent datum on E relative to the fppf covering \{ F \to G\} . By More on Morphisms, Lemma 37.57.1 this descent datum is effective. By Descent, Lemma 35.39.1 again this implies that H is a scheme. By Descent, Lemmas 35.23.6 and 35.23.24 it now follows that b is separated and locally quasi-finite.
\square
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