Lemma 35.3.8. Let $R \to A$ be a faithfully flat ring map, and let $R \to R'$ be faithfully flat. Set $A' = R' \otimes _ R A$. If all descent data for $R' \to A'$ are effective, then so are all descent data for $R \to A$.

Proof. Let $(N, \varphi )$ be a descent datum for $R \to A$. Set $N' = R' \otimes _ R N = A' \otimes _ A N$, and denote $\varphi ' = \text{id}_{R'} \otimes \varphi$ the base change of the descent datum $\varphi$. Then $(N', \varphi ')$ is a descent datum for $R' \to A'$ and $H^0(s(N'_\bullet )) = R' \otimes _ R H^0(s(N_\bullet ))$. Moreover, the map $A' \otimes _{R'} H^0(s(N'_\bullet )) \to N'$ is identified with the base change of the $A$-module map $A \otimes _ R H^0(s(N)) \to N$ via the faithfully flat map $A \to A'$. Hence we conclude by Lemma 35.3.7. $\square$

Comment #2874 by Ko Aoki on

Possibly wrong statement: The proof uses 039W, but it holds under an additional condition that $R \to A$ is faithfully flat. So I think faithfully flatness of $R \to A$ is needed. The same for 039Y.

Comment #2952 by Ko Aoki on

Thank you for fixing. (1) of 039Y contains the same error.

Comment #3079 by on

@#2952 OK, I am very sorry. When I fixed the first mistake I looked (using the statistics feature of the website) where the lemma is used but the remark didn't show up. Now this is fixed too, see here. Thanks!

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