## 76.13 Grothendieck's Existence Theorem, bis

In this section we prove an analogue for Grothendieck's existence theorem in the derived category, following the method used in Section 76.12 for quasi-coherent modules. This section is the analogue of More on Flatness, Section 38.29 for algebraic spaces. The classical case (for algebraic spaces) is discussed in More on Morphisms of Spaces, Section 75.42. We will work in the following situation.

Situation 76.13.1. Here we have an inverse system of rings $(A_ n)$ with surjective transition maps whose kernels are locally nilpotent. Set $A = \mathop{\mathrm{lim}}\nolimits A_ n$. We have an algebraic space $X$ proper, flat, and of finite presentation over $A$. We set $X_ n = X \times _{\mathop{\mathrm{Spec}}(A)} \mathop{\mathrm{Spec}}(A_ n)$ and we view it as a closed subspace of $X$. We assume further given a system $(K_ n, \varphi _ n)$ where $K_ n$ is a pseudo-coherent object of $D(\mathcal{O}_{X_ n})$ and

\[ \varphi _ n : K_ n \longrightarrow K_{n - 1} \]

is a map in $D(\mathcal{O}_{X_ n})$ which induces an isomorphism $K_ n \otimes _{\mathcal{O}_{X_ n}}^\mathbf {L} \mathcal{O}_{X_{n - 1}} \to K_{n - 1}$ in $D(\mathcal{O}_{X_{n - 1}})$.

More precisely, we should write $\varphi _ n : K_ n \to Ri_{n - 1, *}K_{n - 1}$ where $i_{n - 1} : X_{n - 1} \to X_ n$ is the inclusion morphism and in this notation the condition is that the adjoint map $Li_{n - 1}^*K_ n \to K_{n - 1}$ is an isomorphism. Our goal is to find a pseudo-coherent $K \in D(\mathcal{O}_ X)$ such that $K_ n = K \otimes _{\mathcal{O}_ X}^\mathbf {L} \mathcal{O}_{X_ n}$ for all $n$ (with the same abuse of notation).

Lemma 76.13.2. In Situation 76.13.1 consider

\[ K = R\mathop{\mathrm{lim}}\nolimits _{D_\mathit{QCoh}(\mathcal{O}_ X)}(K_ n) = DQ_ X(R\mathop{\mathrm{lim}}\nolimits _{D(\mathcal{O}_ X)} K_ n) \]

Then $K$ is in $D^-_{\mathit{QCoh}}(\mathcal{O}_ X)$.

**Proof.**
The functor $DQ_ X$ exists because $X$ is quasi-compact and quasi-separated, see Derived Categories of Spaces, Lemma 74.19.1. Since $DQ_ X$ is a right adjoint it commutes with products and therefore with derived limits. Hence the equality in the statement of the lemma.

By Derived Categories of Spaces, Lemma 74.19.4 the functor $DQ_ X$ has bounded cohomological dimension. Hence it suffices to show that $R\mathop{\mathrm{lim}}\nolimits K_ n \in D^-(\mathcal{O}_ X)$. To see this, let $U \to X$ be étale with $U$ affine. Then there is a canonical exact sequence

\[ 0 \to R^1\mathop{\mathrm{lim}}\nolimits H^{m - 1}(U, K_ n) \to H^ m(U, R\mathop{\mathrm{lim}}\nolimits K_ n) \to \mathop{\mathrm{lim}}\nolimits H^ m(U, K_ n) \to 0 \]

by Cohomology on Sites, Lemma 21.23.2. Since $U$ is affine and $K_ n$ is pseudo-coherent (and hence has quasi-coherent cohomology sheaves by Derived Categories of Spaces, Lemma 74.13.6) we see that $H^ m(U, K_ n) = H^ m(K_ n)(U)$ by Derived Categories of Schemes, Lemma 36.3.5. Thus we conclude that it suffices to show that $K_ n$ is bounded above independent of $n$.

Since $K_ n$ is pseudo-coherent we have $K_ n \in D^-(\mathcal{O}_{X_ n})$. Suppose that $a_ n$ is maximal such that $H^{a_ n}(K_ n)$ is nonzero. Of course $a_1 \leq a_2 \leq a_3 \leq \ldots $. Note that $H^{a_ n}(K_ n)$ is an $\mathcal{O}_{X_ n}$-module of finite presentation (Cohomology on Sites, Lemma 21.45.7). We have $H^{a_ n}(K_{n - 1}) = H^{a_ n}(K_ n) \otimes _{\mathcal{O}_{X_ n}} \mathcal{O}_{X_{n - 1}}$. Since $X_{n - 1} \to X_ n$ is a thickening, it follows from Nakayama's lemma (Algebra, Lemma 10.20.1) that if $H^{a_ n}(K_ n) \otimes _{\mathcal{O}_{X_ n}} \mathcal{O}_{X_{n - 1}}$ is zero, then $H^{a_ n}(K_ n)$ is zero too (argue by checking on stalks for example; small detail omitted). Thus $a_{n - 1} = a_ n$ for all $n$ and we conclude.
$\square$

Lemma 76.13.3. In Situation 76.13.1 let $K$ be as in Lemma 76.13.2. For any perfect object $E$ of $D(\mathcal{O}_ X)$ the cohomology

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

is a pseudo-coherent object of $D(A)$ and there is a canonical isomorphism

\[ R\Gamma (X_ n, K_ n \otimes ^\mathbf {L} E|_{X_ n}) = M \otimes _ A^\mathbf {L} A_ n \]

in $D(A_ n)$. Here $E|_{X_ n}$ denotes the derived pullback of $E$ to $X_ n$.

**Proof.**
Write $E_ n = E|_{X_ n}$ and $M_ n = R\Gamma (X_ n, K_ n \otimes ^\mathbf {L} E|_{X_ n})$. By Derived Categories of Spaces, Lemma 74.25.5 we see that $M_ n$ is a pseudo-coherent object of $D(A_ n)$ whose formation commutes with base change. Thus the maps $M_ n \otimes _{A_ n}^\mathbf {L} A_{n - 1} \to M_{n - 1}$ coming from $\varphi _ n$ are isomorphisms. By More on Algebra, Lemma 15.97.1 we find that $R\mathop{\mathrm{lim}}\nolimits M_ n$ is pseudo-coherent and that its base change back to $A_ n$ recovers $M_ n$. On the other hand, the exact functor $R\Gamma (X, -) : D_\mathit{QCoh}(\mathcal{O}_ X) \to D(A)$ of triangulated categories commutes with products and hence with derived limits, whence

\[ R\Gamma (X, E \otimes ^\mathbf {L} K) = R\mathop{\mathrm{lim}}\nolimits R\Gamma (X, E \otimes ^\mathbf {L} K_ n) = R\mathop{\mathrm{lim}}\nolimits R\Gamma (X_ n, E_ n \otimes ^\mathbf {L} K_ n) = R\mathop{\mathrm{lim}}\nolimits M_ n \]

as desired.
$\square$

Lemma 76.13.4. In Situation 76.13.1 let $K$ be as in Lemma 76.13.2. Then $K$ is pseudo-coherent on $X$.

**Proof.**
Combinging Lemma 76.13.3 and Derived Categories of Spaces, Lemma 74.25.7 we see that $R\Gamma (X, K \otimes ^\mathbf {L} E)$ is pseudo-coherent in $D(A)$ for all pseudo-coherent $E$ in $D(\mathcal{O}_ X)$. Thus it follows from More on Morphisms of Spaces, Lemma 75.51.4 that $K$ is pseudo-coherent relative to $A$. Since $X$ is of flat and of finite presentation over $A$, this is the same as being pseudo-coherent on $X$, see More on Morphisms of Spaces, Lemma 75.45.4.
$\square$

Lemma 76.13.5. In Situation 76.13.1 let $K$ be as in Lemma 76.13.2. For any étale morphism $U \to X$ with $U$ quasi-compact and quasi-separated we have

\[ R\Gamma (U, K) \otimes _ A^\mathbf {L} A_ n = R\Gamma (U_ n, K_ n) \]

in $D(A_ n)$ where $U_ n = U \times _ X X_ n$.

**Proof.**
Fix $n$. By Derived Categories of Spaces, Lemma 74.27.3 there exists a system of perfect complexes $E_ m$ on $X$ such that $R\Gamma (U, K) = \text{hocolim} R\Gamma (X, K \otimes ^\mathbf {L} E_ m)$. In fact, this formula holds not just for $K$ but for every object of $D_\mathit{QCoh}(\mathcal{O}_ X)$. Applying this to $K_ n$ we obtain

\begin{align*} R\Gamma (U_ n, K_ n) & = R\Gamma (U, K_ n) \\ & = \text{hocolim}_ m R\Gamma (X, K_ n \otimes ^\mathbf {L} E_ m) \\ & = \text{hocolim}_ m R\Gamma (X_ n, K_ n \otimes ^\mathbf {L} E_ m|_{X_ n}) \end{align*}

Using Lemma 76.13.3 and the fact that $- \otimes _ A^\mathbf {L} A_ n$ commutes with homotopy colimits we obtain the result.
$\square$

Theorem 76.13.6 (Derived Grothendieck Existence Theorem). In Situation 76.13.1 there exists a pseudo-coherent $K$ in $D(\mathcal{O}_ X)$ such that $K_ n = K \otimes _{\mathcal{O}_ X}^\mathbf {L} \mathcal{O}_{X_ n}$ for all $n$ compatibly with the maps $\varphi _ n$.

**Proof.**
Apply Lemmas 76.13.2, 76.13.3, 76.13.4 to get a pseudo-coherent object $K$ of $D(\mathcal{O}_ X)$. Choosing affine $U$ in Lemma 76.13.5 it follows immediately that $K$ restricts to $K_ n$ over $X_ n$.
$\square$

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