The Stacks project

Lemma 73.25.7. Let $A$ be a ring. Let $X$ be an algebraic space over $A$ which is quasi-compact and quasi-separated. Let $K \in D^-_\mathit{QCoh}(\mathcal{O}_ X)$. If $R\Gamma (X, E \otimes ^\mathbf {L} K)$ is pseudo-coherent in $D(A)$ for every perfect $E$ in $D(\mathcal{O}_ X)$, then $R\Gamma (X, E \otimes ^\mathbf {L} K)$ is pseudo-coherent in $D(A)$ for every pseudo-coherent $E$ in $D(\mathcal{O}_ X)$.

Proof. There exists an integer $N$ such that $R\Gamma (X, -) : D_\mathit{QCoh}(\mathcal{O}_ X) \to D(A)$ has cohomological dimension $N$ as explained in Lemma 73.6.1. Let $b \in \mathbf{Z}$ be such that $H^ i(K) = 0$ for $i > b$. Let $E$ be pseudo-coherent on $X$. It suffices to show that $R\Gamma (X, E \otimes ^\mathbf {L} K)$ is $m$-pseudo-coherent for every $m$. Choose an approximation $P \to E$ by a perfect complex $P$ of $(X, E, m - N - 1 - b)$. This is possible by Theorem 73.14.7. Choose a distinguished triangle

\[ P \to E \to C \to P[1] \]

in $D_\mathit{QCoh}(\mathcal{O}_ X)$. The cohomology sheaves of $C$ are zero in degrees $\geq m - N - 1 - b$. Hence the cohomology sheaves of $C \otimes ^\mathbf {L} K$ are zero in degrees $\geq m - N - 1$. Thus the cohomology of $R\Gamma (X, C \otimes ^\mathbf {L} K)$ are zero in degrees $\geq m - 1$. Hence

\[ R\Gamma (X, P \otimes ^\mathbf {L} K) \to R\Gamma (X, E \otimes ^\mathbf {L} K) \]

is an isomorphism on cohomology in degrees $\geq m$. By assumption the source is pseudo-coherent. We conclude that $R\Gamma (X, E \otimes ^\mathbf {L} K)$ is $m$-pseudo-coherent as desired. $\square$


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