The Stacks project

Proof. Let $Q \in D^+_\mathit{QCoh}(\mathcal{O}_ Y)$ be supported on $Y \setminus V$. To get a contradiction, assume that $a(Q)$ is not supported on $X \setminus U$. Then we can find a perfect complex $P_ U$ on $U$ and a nonzero map $P_ U \to a(Q)|_ U$ (follows from Derived Categories of Schemes, Theorem 36.15.3). Then using Derived Categories of Schemes, Lemma 36.13.11 we may assume there is a perfect complex $P$ on $X$ and a map $P \to a(Q)$ whose restriction to $U$ is nonzero. By definition of $a$ this map is adjoint to a map $Rf_*P \to Q$.

The complex $Rf_*P$ is pseudo-coherent. In case (1) this follows from Derived Categories of Schemes, Lemma 36.30.5. In case (2) this follows from Derived Categories of Schemes, Lemmas 36.11.3 and 36.10.3. Thus we may apply Derived Categories of Schemes, Lemma 36.17.5 and get a map $I \to \mathcal{O}_ Y$ of perfect complexes whose restriction to $V$ is an isomorphism such that the composition $I \otimes ^\mathbf {L}_{\mathcal{O}_ Y} Rf_*P \to Rf_*P \to Q$ is zero. By Derived Categories of Schemes, Lemma 36.22.1 we have $I \otimes ^\mathbf {L}_{\mathcal{O}_ Y} Rf_*P = Rf_*(Lf^*I \otimes ^\mathbf {L}_{\mathcal{O}_ X} P)$. We conclude that the composition

is zero. However, the restriction to $U$ is the map $P|_ U \to a(Q)|_ U$ which we assumed to be nonzero. This contradiction finishes the proof. $\square$