Lemma 33.30.1. Let A \to B be a faithfully flat ring map. Let X be a quasi-compact and quasi-separated scheme over A. Let \mathcal{L} be an invertible \mathcal{O}_ X-module whose pullback to X_ B is trivial. Then H^0(X, \mathcal{L}) and H^0(X, \mathcal{L}^{\otimes -1}) are invertible H^0(X, \mathcal{O}_ X)-modules and the multiplication map induces an isomorphism
Proof. Denote \mathcal{L}_ B the pullback of \mathcal{L} to X_ B. Choose an isomorphism \mathcal{L}_ B \to \mathcal{O}_{X_ B}. Set R = H^0(X, \mathcal{O}_ X), M = H^0(X, \mathcal{L}) and think of M as an R-module. For every quasi-coherent \mathcal{O}_ X-module \mathcal{F} with pullback \mathcal{F}_ B on X_ B there is a canonical isomorphism H^0(X_ B, \mathcal{F}_ B) = H^0(X, \mathcal{F}) \otimes _ A B, see Cohomology of Schemes, Lemma 30.5.2. Thus we have
Since R \to R \otimes _ A B is faithfully flat (as the base change of the faithfully flat map A \to B), we conclude that M is an invertible R-module by Algebra, Proposition 10.83.3. Similarly N = H^0(X, \mathcal{L}^{\otimes -1}) is an invertible R-module. To see that the statement on tensor products is true, use that it is true after pulling back to X_ B and faithful flatness of R \to R \otimes _ A B. Some details omitted. \square
Comments (0)