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The Stacks project

Lemma 61.20.2. Let \Lambda be a Noetherian ring. Let I \subset \Lambda be an ideal. Let f : \mathop{\mathit{Sh}}\nolimits (\mathcal{D}) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C}) be a morphism of topoi. Then

  1. Rf_* sends D_{comp}(\mathcal{D}, \Lambda ) into D_{comp}(\mathcal{C}, \Lambda ),

  2. the map Rf_* : D_{comp}(\mathcal{D}, \Lambda ) \to D_{comp}(\mathcal{C}, \Lambda ) has a left adjoint Lf_{comp}^* : D_{comp}(\mathcal{C}, \Lambda ) \to D_{comp}(\mathcal{D}, \Lambda ) which is Lf^* followed by derived completion,

  3. Rf_* commutes with derived completion,

  4. for K in D_{comp}(\mathcal{D}, \Lambda ) we have Rf_*K = R\mathop{\mathrm{lim}}\nolimits Rf_*(K \otimes ^\mathbf {L}_\Lambda \underline{\Lambda /I^ n}).

  5. for M in D_{comp}(\mathcal{C}, \Lambda ) we have Lf^*_{comp}M = R\mathop{\mathrm{lim}}\nolimits Lf^*(M \otimes ^\mathbf {L}_\Lambda \underline{\Lambda /I^ n}).

Proof. We have seen (1) and (2) in Algebraic and Formal Geometry, Lemma 52.6.18. Part (3) follows from Algebraic and Formal Geometry, Lemma 52.6.19. For (4) let K be derived complete. Then

Rf_*K = Rf_*( R\mathop{\mathrm{lim}}\nolimits K \otimes ^\mathbf {L}_\Lambda \underline{\Lambda /I^ n}) = R\mathop{\mathrm{lim}}\nolimits Rf_*(K \otimes ^\mathbf {L}_\Lambda \underline{\Lambda /I^ n})

the first equality by Lemma 61.20.1 and the second because Rf_* commutes with R\mathop{\mathrm{lim}}\nolimits (Cohomology on Sites, Lemma 21.23.3). This proves (4). To prove (5), by Lemma 61.20.1 we have

Lf_{comp}^*M = R\mathop{\mathrm{lim}}\nolimits ( Lf^*M \otimes _\Lambda ^\mathbf {L} \underline{\Lambda /I^ n})

Since Lf^* commutes with derived tensor product by Cohomology on Sites, Lemma 21.18.4 and since Lf^*\underline{\Lambda /I^ n} = \underline{\Lambda /I^ n} we get (5). \square


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