The Stacks project

Lemma 36.30.4. Let $S$ be a scheme. Let $f : X \to S$ be a proper morphism of finite presentation.

  1. Let $E \in D(\mathcal{O}_ X)$ be perfect and $f$ flat. Then $Rf_*E$ is a perfect object of $D(\mathcal{O}_ S)$ and its formation commutes with arbitrary base change.

  2. Let $\mathcal{G}$ be an $\mathcal{O}_ X$-module of finite presentation, flat over $S$. Then $Rf_*\mathcal{G}$ is a perfect object of $D(\mathcal{O}_ S)$ and its formation commutes with arbitrary base change.

Proof. Special cases of Lemma 36.30.1 applied with (1) $\mathcal{G}^\bullet $ equal to $\mathcal{O}_ X$ in degree $0$ and (2) $E = \mathcal{O}_ X$ and $\mathcal{G}^\bullet $ consisting of $\mathcal{G}$ sitting in degree $0$. $\square$

Comments (2)

Comment #4352 by Remy on

In (2), does not need to be flat.

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