Lemma 20.48.2. Let $X$ be a ringed space. Assume that the underlying topological space of $X$ has the following properties:

1. $X$ is quasi-compact,

2. there exists a basis of quasi-compact open subsets, and

3. the intersection of any two quasi-compact opens is quasi-compact.

Let $K$ be a perfect object of $D(\mathcal{O}_ X)$. Then

1. $K$ is a compact object of $D^+(\mathcal{O}_ X)$ in the following sense: if $M = \bigoplus _{i \in I} M_ i$ is bounded below, then $\mathop{\mathrm{Hom}}\nolimits (K, M) = \bigoplus _{i \in I} \mathop{\mathrm{Hom}}\nolimits (K, M_ i)$.

2. If $X$ has finite cohomological dimension, i.e., if there exists a $d$ such that $H^ i(X, \mathcal{F}) = 0$ for $i > d$, then $K$ is a compact object of $D(\mathcal{O}_ X)$.

Proof. Let $K^\vee$ be the dual of $K$, see Lemma 20.46.4. Then we have

$\mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_ X)}(K, M) = H^0(X, K^\vee \otimes _{\mathcal{O}_ X}^\mathbf {L} M)$

functorially in $M$ in $D(\mathcal{O}_ X)$. Since $K^\vee \otimes _{\mathcal{O}_ X}^\mathbf {L} -$ commutes with direct sums it suffices to show that $R\Gamma (X, -)$ commutes with the relevant direct sums.

Proof of (b). Since $R\Gamma (X, K) = R\mathop{\mathrm{Hom}}\nolimits (\mathcal{O}_ X, K)$ and since $H^ p(X, -)$ commutes with direct sums by Lemma 20.19.1 this is a special case of Lemma 20.48.1

Proof of (a). Let $\mathcal{I}_ i$, $i \in I$ be a collection of injective $\mathcal{O}_ X$-modules. By Lemma 20.19.1 we see that

$H^ p(X, \bigoplus \nolimits _{i \in I} \mathcal{I}_ i) = \bigoplus \nolimits _{i \in I} H^ p(X, \mathcal{I}_ i) = 0$

for all $p$. Now if $M = \bigoplus M_ i$ is as in (a), then we see that there exists an $a \in \mathbf{Z}$ such that $H^ n(M_ i) = 0$ for $n < a$. Thus we can choose complexes of injective $\mathcal{O}_ X$-modules $\mathcal{I}_ i^\bullet$ representing $M_ i$ with $\mathcal{I}_ i^ n = 0$ for $n < a$, see Derived Categories, Lemma 13.18.3. By Injectives, Lemma 19.13.4 we see that the direct sum complex $\bigoplus \mathcal{I}_ i^\bullet$ represents $M$. By Leray acyclicity (Derived Categories, Lemma 13.16.7) we see that

$R\Gamma (X, M) = \Gamma (X, \bigoplus \mathcal{I}_ i^\bullet ) = \bigoplus \Gamma (X, \bigoplus \mathcal{I}_ i^\bullet ) = \bigoplus R\Gamma (X, M_ i)$

as desired. $\square$

Comment #4080 by on

Jack Hall just pointed out that this lemma is wrong. It is correct if X has finite coh dim or if we look at $D^+$. I will fix it soon.

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