The Stacks project

57.72 More on constructible sheaves

Let $\Lambda $ be a Noetherian ring. Let $X$ be a scheme. We often consider $X_{\acute{e}tale}$ as a ringed site with sheaf of rings $\underline{\Lambda }$. In case of abelian sheaves we often take $\Lambda = \mathbf{Z}/n\mathbf{Z}$ for a suitable integer $n$.

Lemma 57.72.1. Let $j : U \to X$ be an étale morphism of quasi-compact and quasi-separated schemes.

  1. The sheaf $h_ U$ is a constructible sheaf of sets.

  2. The sheaf $j_!\underline{M}$ is a constructible abelian sheaf for a finite abelian group $M$.

  3. If $\Lambda $ is a Noetherian ring and $M$ is a finite $\Lambda $-module, then $j_!\underline{M}$ is a constructible sheaf of $\Lambda $-modules on $X_{\acute{e}tale}$.

Proof. By Lemma 57.71.1 there is a partition $\coprod _ i X_ i$ such that $\pi _ i : j^{-1}(X_ i) \to X_ i$ is finite étale. The restriction of $h_ U$ to $X_ i$ is $h_{j^{-1}(X_ i)}$ which is finite locally constant by Lemma 57.63.4. For cases (2) and (3) we note that

\[ j_!(\underline{M})|_{X_ i} = \pi _{i!}(\underline{M}) = \pi _{i*}(\underline{M}) \]

by Lemmas 57.69.5 and 57.69.7. Thus it suffices to show the lemma for $\pi : Y \to X$ finite étale. This is Lemma 57.63.3. $\square$

Lemma 57.72.2. Let $X$ be a quasi-compact and quasi-separated scheme.

  1. Let $\mathcal{F}$ be a sheaf of sets on $X_{\acute{e}tale}$. Then $\mathcal{F}$ is a filtered colimit of constructible sheaves of sets.

  2. Let $\mathcal{F}$ be a torsion abelian sheaf on $X_{\acute{e}tale}$. Then $\mathcal{F}$ is a filtered colimit of constructible abelian sheaves.

  3. Let $\Lambda $ be a Noetherian ring and $\mathcal{F}$ a sheaf of $\Lambda $-modules on $X_{\acute{e}tale}$. Then $\mathcal{F}$ is a filtered colimit of constructible sheaves of $\Lambda $-modules.

Proof. Let $\mathcal{B}$ be the collection of quasi-compact and quasi-separated objects of $X_{\acute{e}tale}$. By Modules on Sites, Lemma 18.30.6 any sheaf of sets is a filtered colimit of sheaves of the form

\[ \text{Coequalizer}\left( \xymatrix{ \coprod \nolimits _{j = 1, \ldots , m} h_{V_ j} \ar@<1ex>[r] \ar@<-1ex>[r] & \coprod \nolimits _{i = 1, \ldots , n} j_{U_ i} } \right) \]

with $V_ j$ and $U_ i$ quasi-compact and quasi-separated objects of $X_{\acute{e}tale}$. By Lemmas 57.72.1 and 57.70.6 these coequalizers are constructible. This proves (1).

Let $\Lambda $ be a Noetherian ring. By Modules on Sites, Lemma 18.30.6 $\Lambda $-modules $\mathcal{F}$ is a filtered colimit of modules of the form

\[ \mathop{\mathrm{Coker}}\left( \bigoplus \nolimits _{j = 1, \ldots , m} j_{V_ j!}\underline{\Lambda }_{V_ j} \longrightarrow \bigoplus \nolimits _{i = 1, \ldots , n} j_{U_ i!}\underline{\Lambda }_{U_ i} \right) \]

with $V_ j$ and $U_ i$ quasi-compact and quasi-separated objects of $X_{\acute{e}tale}$. By Lemmas 57.72.1 and 57.70.6 these cokernels are constructible. This proves (3).

Proof of (2). First write $\mathcal{F} = \bigcup \mathcal{F}[n]$ where $\mathcal{F}[n]$ is the $n$-torsion subsheaf. Then we can view $\mathcal{F}[n]$ as a sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules and apply (3). $\square$

Lemma 57.72.3. Let $f : X \to Y$ be a surjective morphism of quasi-compact and quasi-separated schemes.

  1. Let $\mathcal{F}$ be a sheaf of sets on $Y_{\acute{e}tale}$. Then $\mathcal{F}$ is constructible if and only if $f^{-1}\mathcal{F}$ is constructible.

  2. Let $\mathcal{F}$ be an abelian sheaf on $Y_{\acute{e}tale}$. Then $\mathcal{F}$ is constructible if and only if $f^{-1}\mathcal{F}$ is constructible.

  3. Let $\Lambda $ be a Noetherian ring. Let $\mathcal{F}$ be sheaf of $\Lambda $-modules on $Y_{\acute{e}tale}$. Then $\mathcal{F}$ is constructible if and only if $f^{-1}\mathcal{F}$ is constructible.

Proof. One implication follows from Lemma 57.70.5. For the converse, assume $f^{-1}\mathcal{F}$ is constructible. Write $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits \mathcal{F}_ i$ as a filtered colimit of constructible sheaves (of sets, abelian groups, or modules) using Lemma 57.72.2. Since $f^{-1}$ is a left adjoint it commutes with colimits (Categories, Lemma 4.24.5) and we see that $f^{-1}\mathcal{F} = \mathop{\mathrm{colim}}\nolimits f^{-1}\mathcal{F}_ i$. By Lemma 57.70.9 we see that $f^{-1}\mathcal{F}_ i \to f^{-1}\mathcal{F}$ is surjective for all $i$ large enough. Since $f$ is surjective we conclude (by looking at stalks using Lemma 57.36.2 and Theorem 57.29.10) that $\mathcal{F}_ i \to \mathcal{F}$ is surjective for all $i$ large enough. Thus $\mathcal{F}$ is the quotient of a constructible sheaf $\mathcal{G}$. Applying the argument once more to $\mathcal{G} \times _\mathcal {F} \mathcal{G}$ or the kernel of $\mathcal{G} \to \mathcal{F}$ we conclude using that $f^{-1}$ is exact and that the category of constructible sheaves (of sets, abelian groups, or modules) is preserved under finite (co)limits or (co)kernels inside $\mathop{\mathit{Sh}}\nolimits (Y_{\acute{e}tale})$, $\mathop{\mathit{Sh}}\nolimits (X_{\acute{e}tale})$, $\textit{Ab}(Y_{\acute{e}tale})$, $\textit{Ab}(X_{\acute{e}tale})$, $\textit{Mod}(Y_{\acute{e}tale}, \Lambda )$, and $\textit{Mod}(X_{\acute{e}tale}, \Lambda )$, see Lemma 57.70.6. $\square$

Lemma 57.72.4. Let $f : X \to Y$ be a finite étale morphism of schemes. Let $\Lambda $ be a Noetherian ring. If $\mathcal{F}$ is a constructible sheaf of sets, constructible sheaf of abelian groups, or constructible sheaf of $\Lambda $-modules on $X_{\acute{e}tale}$, the same is true for $f_*\mathcal{F}$ on $Y_{\acute{e}tale}$.

Proof. By Lemma 57.70.4 it suffices to check this Zariski locally on $Y$ and by Lemma 57.72.3 we may replace $Y$ by an étale cover (the construction of $f_*$ commutes with étale localization). A finite étale morphism is étale locally isomorphic to a disjoint union of isomorphisms, see Étale Morphisms, Lemma 41.18.3. Thus, in the case of sheaves of sets, the lemma says that if $\mathcal{F}_ i$, $i = 1, \ldots , n$ are constructible sheaves of sets, then $\prod _{i = 1, \ldots , n} \mathcal{F}_ i$ is too. This is clear. Similarly for sheaves of abelian groups and modules. $\square$

Lemma 57.72.5. Let $X$ be a quasi-compact and quasi-separated scheme. The category of constructible sheaves of sets is the full subcategory of $\mathop{\mathit{Sh}}\nolimits (X_{\acute{e}tale})$ consisting of sheaves $\mathcal{F}$ which are coequalizers

\[ \xymatrix{ \mathcal{F}_1 \ar@<1ex>[r] \ar@<-1ex>[r] & \mathcal{F}_0 \ar[r] & \mathcal{F}} \]

such that $\mathcal{F}_ i$, $i = 0, 1$ is a finite coproduct of sheaves of the form $h_ U$ with $U$ a quasi-compact and quasi-separated object of $X_{\acute{e}tale}$.

Proof. In the proof of Lemma 57.72.2 we have seen that sheaves of this form are constructible. For the converse, suppose that for every constructible sheaf of sets $\mathcal{F}$ we can find a surjection $\mathcal{F}_0 \to \mathcal{F}$ with $\mathcal{F}_0$ as in the lemma. Then we find our surjection $\mathcal{F}_1 \to \mathcal{F}_0 \times _\mathcal {F} \mathcal{F}_0$ because the latter is constructible by Lemma 57.70.6.

By Topology, Lemma 5.28.7 we may choose a finite stratification $X = \coprod _{i \in I} X_ i$ such that $\mathcal{F}$ is finite locally constant on each stratum. We will prove the result by induction on the cardinality of $I$. Let $i \in I$ be a minimal element in the partial ordering of $I$. Then $X_ i \subset X$ is closed. By induction, there exist finitely many quasi-compact and quasi-separated objects $U_\alpha $ of $(X \setminus X_ i)_{\acute{e}tale}$ and a surjective map $\coprod h_{U_\alpha } \to \mathcal{F}|_{X \setminus X_ i}$. These determine a map

\[ \coprod h_{U_\alpha } \to \mathcal{F} \]

which is surjective after restricting to $X \setminus X_ i$. By Lemma 57.63.4 we see that $\mathcal{F}|_{X_ i} = h_ V$ for some scheme $V$ finite étale over $X_ i$. Let $\overline{v}$ be a geometric point of $V$ lying over $\overline{x} \in X_ i$. We may think of $\overline{v}$ as an element of the stalk $\mathcal{F}_{\overline{x}} = V_{\overline{x}}$. Thus we can find an étale neighbourhood $(U, \overline{u})$ of $\overline{x}$ and a section $s \in \mathcal{F}(U)$ whose stalk at $\overline{x}$ gives $\overline{v}$. Thinking of $s$ as a map $s : h_ U \to \mathcal{F}$, restricting to $X_ i$ we obtain a morphism $s|_{X_ i} : U \times _ X X_ i \to V$ over $X_ i$ which maps $\overline{u}$ to $\overline{v}$. Since $V$ is quasi-compact (finite over the closed subscheme $X_ i$ of the quasi-compact scheme $X$) a finite number $s^{(1)}, \ldots , s^{(m)}$ of these sections of $\mathcal{F}$ over $U^{(1)}, \ldots , U^{(m)}$ will determine a jointly surjective map

\[ \coprod s^{(j)}|_{X_ i} : \coprod U^{(j)} \times _ X X_ i \longrightarrow V \]

Then we obtain the surjection

\[ \coprod h_{U_\alpha } \amalg \coprod h_{U^{(j)}} \to \mathcal{F} \]

as desired. $\square$

Lemma 57.72.6. Let $X$ be a quasi-compact and quasi-separated scheme. Let $\Lambda $ be a Noetherian ring. The category of constructible sheaves of $\Lambda $-modules is exactly the category of modules of the form

\[ \mathop{\mathrm{Coker}}\left( \bigoplus \nolimits _{j = 1, \ldots , m} j_{V_ j!}\underline{\Lambda }_{V_ j} \longrightarrow \bigoplus \nolimits _{i = 1, \ldots , n} j_{U_ i!}\underline{\Lambda }_{U_ i} \right) \]

with $V_ j$ and $U_ i$ quasi-compact and quasi-separated objects of $X_{\acute{e}tale}$. In fact, we can even assume $U_ i$ and $V_ j$ affine.

Proof. In the proof of Lemma 57.72.2 we have seen modules of this form are constructible. Since the category of constructible modules is abelian (Lemma 57.70.6) it suffices to prove that given a constructible module $\mathcal{F}$ there is a surjection

\[ \bigoplus \nolimits _{i = 1, \ldots , n} j_{U_ i!}\underline{\Lambda }_{U_ i} \longrightarrow \mathcal{F} \]

for some affine objects $U_ i$ in $X_{\acute{e}tale}$. By Modules on Sites, Lemma 18.30.6 there is a surjection

\[ \Psi : \bigoplus \nolimits _{i \in I} j_{U_ i!}\underline{\Lambda }_{U_ i} \longrightarrow \mathcal{F} \]

with $U_ i$ affine and the direct sum over a possibly infinite index set $I$. For every finite subset $I' \subset I$ set

\[ T_{I'} = \text{Supp}(\mathop{\mathrm{Coker}}( \bigoplus \nolimits _{i \in I'} j_{U_ i!}\underline{\Lambda }_{U_ i} \longrightarrow \mathcal{F})) \]

By the very definition of constructible sheaves, the set $T_{I'}$ is a constructible subset of $X$. We want to show that $T_{I'} = \emptyset $ for some $I'$. Since every stalk $\mathcal{F}_{\overline{x}}$ is a finite type $\Lambda $-module and since $\Psi $ is surjective, for every $x \in X$ there is an $I'$ such that $x \not\in T_{I'}$. In other words we have $\emptyset = \bigcap _{I' \subset I\text{ finite}} T_{I'}$. Since $X$ is a spectral space by Properties, Lemma 28.2.4 the constructible topology on $X$ is quasi-compact by Topology, Lemma 5.23.2. Thus $T_{I'} = \emptyset $ for some $I' \subset I$ finite as desired. $\square$

Lemma 57.72.7. Let $X$ be a quasi-compact and quasi-separated scheme. The category of constructible abelian sheaves is exactly the category of abelian sheaves of the form

\[ \mathop{\mathrm{Coker}}\left( \bigoplus \nolimits _{j = 1, \ldots , m} j_{V_ j!}\underline{\mathbf{Z}/m_ j\mathbf{Z}}_{V_ j} \longrightarrow \bigoplus \nolimits _{i = 1, \ldots , n} j_{U_ i!}\underline{\mathbf{Z}/n_ i\mathbf{Z}}_{U_ i} \right) \]

with $V_ j$ and $U_ i$ quasi-compact and quasi-separated objects of $X_{\acute{e}tale}$ and $m_ j$, $n_ i$ positive integers. In fact, we can even assume $U_ i$ and $V_ j$ affine.

Proof. This follows from Lemma 57.72.6 applied with $\Lambda = \mathbf{Z}/n\mathbf{Z}$ and the fact that, since $X$ is quasi-compact, every constructible abelian sheaf is annihilated by some positive integer $n$ (details omitted). $\square$

Lemma 57.72.8. Let $X$ be a quasi-compact and quasi-separated scheme. Let $\Lambda $ be a Noetherian ring. Let $\mathcal{F}$ be a constructible sheaf of sets, abelian groups, or $\Lambda $-modules on $X_{\acute{e}tale}$. Let $\mathcal{G} = \mathop{\mathrm{colim}}\nolimits \mathcal{G}_ i$ be a filtered colimit of sheaves of sets, abelian groups, or $\Lambda $-modules. Then

\[ \mathop{Mor}\nolimits (\mathcal{F}, \mathcal{G}) = \mathop{\mathrm{colim}}\nolimits \mathop{Mor}\nolimits (\mathcal{F}, \mathcal{G}_ i) \]

in the category of sheaves of sets, abelian groups, or $\Lambda $-modules on $X_{\acute{e}tale}$.

Proof. The case of sheaves of sets. By Lemma 57.72.5 it suffices to prove the lemma for $h_ U$ where $U$ is a quasi-compact and quasi-separated object of $X_{\acute{e}tale}$. Recall that $\mathop{Mor}\nolimits (h_ U, \mathcal{G}) = \mathcal{G}(U)$. Hence the result follows from Sites, Lemma 7.17.5.

In the case of abelian sheaves or sheaves of modules, the result follows in the same way using Lemmas 57.72.7 and 57.72.6. For the case of abelian sheaves, we add that $\mathop{Mor}\nolimits (j_{U!}\underline{\mathbf{Z}/n\mathbf{Z}}, \mathcal{G})$ is equal to the $n$-torsion elements of $\mathcal{G}(U)$. $\square$

Lemma 57.72.9. Let $f : X \to Y$ be a finite and finitely presented morphism of schemes. Let $\Lambda $ be a Noetherian ring. If $\mathcal{F}$ is a constructible sheaf of sets, abelian groups, or $\Lambda $-modules on $X_{\acute{e}tale}$, then $f_*\mathcal{F}$ is too.

Proof. It suffices to prove this when $X$ and $Y$ are affine by Lemma 57.70.4. By Lemmas 57.54.3 and 57.72.3 we may base change to any affine scheme surjective over $X$. By Lemma 57.71.3 this reduces us to the case of a finite étale morphism (because a thickening leads to an equivalence of étale topoi and even small étale sites, see Theorem 57.45.2). The finite étale case is Lemma 57.72.4. $\square$

Lemma 57.72.10. Let $X = \mathop{\mathrm{lim}}\nolimits _{i \in I} X_ i$ be a limit of a directed system of schemes with affine transition morphisms. We assume that $X_ i$ is quasi-compact and quasi-separated for all $i \in I$.

  1. The category of constructible sheaves of sets on $X_{\acute{e}tale}$ is the colimit of the categories of constructible sheaves of sets on $(X_ i)_{\acute{e}tale}$.

  2. The category of constructible abelian sheaves on $X_{\acute{e}tale}$ is the colimit of the categories of constructible abelian sheaves on $(X_ i)_{\acute{e}tale}$.

  3. Let $\Lambda $ be a Noetherian ring. The category of constructible sheaves of $\Lambda $-modules on $X_{\acute{e}tale}$ is the colimit of the categories of constructible sheaves of $\Lambda $-modules on $(X_ i)_{\acute{e}tale}$.

Proof. Proof of (1). Denote $f_ i : X \to X_ i$ the projection maps. There are 3 parts to the proof corresponding to “faithful”, “fully faithful”, and “essentially surjective”.

Faithful. Choose $0 \in I$ and let $\mathcal{F}_0$, $\mathcal{G}_0$ be constructible sheaves on $X_0$. Suppose that $a, b : \mathcal{F}_0 \to \mathcal{G}_0$ are maps such that $f_0^{-1}a = f_0^{-1}b$. Let $E \subset X_0$ be the set of points $x \in X_0$ such that $a_{\overline{x}} = b_{\overline{x}}$. By Lemma 57.70.8 the subset $E \subset X_0$ is constructible. By assumption $X \to X_0$ maps into $E$. By Limits, Lemma 32.4.10 we find an $i \geq 0$ such that $X_ i \to X_0$ maps into $E$. Hence $f_{i0}^{-1}a = f_{i0}^{-1}b$.

Fully faithful. Choose $0 \in I$ and let $\mathcal{F}_0$, $\mathcal{G}_0$ be constructible sheaves on $X_0$. Suppose that $a : f_0^{-1}\mathcal{F}_0 \to f_0^{-1}\mathcal{G}_0$ is a map. We claim there is an $i$ and a map $a_ i : f_{i0}^{-1}\mathcal{F}_0 \to f_{i0}^{-1}\mathcal{G}_0$ which pulls back to $a$ on $X$. By Lemma 57.72.5 we can replace $\mathcal{F}_0$ by a finite coproduct of sheaves represented by quasi-compact and quasi-separated objects of $(X_0)_{\acute{e}tale}$. Thus we have to show: If $U_0 \to X_0$ is such an object of $(X_0)_{\acute{e}tale}$, then

\[ f_0^{-1}\mathcal{G}(U) = \mathop{\mathrm{colim}}\nolimits _{i \geq 0} f_{i0}^{-1}\mathcal{G}(U_ i) \]

where $U = X \times _{X_0} U_0$ and $U_ i = X_ i \times _{X_0} U_0$. This is a special case of Theorem 57.51.3.

Essentially surjective. We have to show every constructible $\mathcal{F}$ on $X$ is isomorphic to $f_ i^{-1}\mathcal{F}$ for some constructible $\mathcal{F}_ i$ on $X_ i$. Applying Lemma 57.72.5 and using the results of the previous two paragraphs, we see that it suffices to prove this for $h_ U$ for some quasi-compact and quasi-separated object $U$ of $X_{\acute{e}tale}$. In this case we have to show that $U$ is the base change of a quasi-compact and quasi-separated scheme étale over $X_ i$ for some $i$. This follows from Limits, Lemmas 32.10.1 and 32.8.10.

Proof of (3). The argument is very similar to the argument for sheaves of sets, but using Lemma 57.72.6 instead of Lemma 57.72.5. Details omitted. Part (2) follows from part (3) because every constructible abelian sheaf over a quasi-compact scheme is a constructible sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules for some $n$. $\square$

Lemma 57.72.11. Let $X$ be an irreducible scheme with generic point $\eta $.

  1. Let $S' \subset S$ be an inclusion of sets. If we have $\underline{S'} \subset \mathcal{G} \subset \underline{S}$ in $\mathop{\mathit{Sh}}\nolimits (X_{\acute{e}tale})$ and $S' = \mathcal{G}_{\overline{\eta }}$, then $\mathcal{G} = \underline{S'}$.

  2. Let $A' \subset A$ be an inclusion of abelian groups. If we have $\underline{A'} \subset \mathcal{G} \subset \underline{A}$ in $\textit{Ab}(X_{\acute{e}tale})$ and $A' = \mathcal{G}_{\overline{\eta }}$, then $\mathcal{G} = \underline{A'}$.

  3. Let $M' \subset M$ be an inclusion of modules over a ring $\Lambda $. If we have $\underline{M'} \subset \mathcal{G} \subset \underline{M}$ in $\textit{Mod}(X_{\acute{e}tale}, \underline{\Lambda })$ and $M' = \mathcal{G}_{\overline{\eta }}$, then $\mathcal{G} = \underline{M'}$.

Proof. This is true because for every étale morphism $U \to X$ with $U \not= \emptyset $ the point $\eta $ is in the image. $\square$

Lemma 57.72.12. Let $X$ be an integral normal scheme with function field $K$. Let $E$ be a set.

  1. Let $g : \mathop{\mathrm{Spec}}(K) \to X$ be the inclusion of the generic point. Then $g_*\underline{E} = \underline{E}$.

  2. Let $j : U \to X$ be the inclusion of a nonempty open. Then $j_*\underline{E} = \underline{E}$.

Proof. Proof of (1). Let $x \in X$ be a point. Let $\mathcal{O}_{X, \overline{x}}$ be a strict henselization of $\mathcal{O}_{X, x}$. By More on Algebra, Lemma 15.44.6 we see that $\mathcal{O}_{X, \overline{x}}$ is a normal domain. Hence $\mathop{\mathrm{Spec}}(K) \times _ X \mathop{\mathrm{Spec}}(\mathcal{O}_{X, \overline{x}})$ is irreducible. It follows that the stalk $(g_*\underline{E}_{\underline{x}}$ is equal to $E$, see Theorem 57.52.1.

Proof of (2). Since $g$ factors through $j$ there is a map $j_*\underline{E} \to g_*\underline{E}$. This map is injective because for every scheme $V$ étale over $X$ the set $\mathop{\mathrm{Spec}}(K) \times _ X V$ is dense in $U \times _ X V$. On the other hand, we have a map $\underline{E} \to j_*\underline{E}$ and we conclude. $\square$

Lemma 57.72.13. Let $X$ be a quasi-compact and quasi-separated scheme. Let $\eta \in X$ be a generic point of an irreducible component of $X$.

  1. Let $\mathcal{F}$ be a torsion abelian sheaf on $X_{\acute{e}tale}$ whose stalk $\mathcal{F}_{\overline{\eta }}$ is zero. Then $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits \mathcal{F}_ i$ is a filtered colimit of constructible abelian sheaves $\mathcal{F}_ i$ such that for each $i$ the support of $\mathcal{F}_ i$ is contained in a closed subscheme not containing $\eta $.

  2. Let $\Lambda $ be a Noetherian ring and $\mathcal{F}$ a sheaf of $\Lambda $-modules on $X_{\acute{e}tale}$ whose stalk $\mathcal{F}_{\overline{\eta }}$ is zero. Then $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits \mathcal{F}_ i$ is a filtered colimit of constructible sheaves of $\Lambda $-modules $\mathcal{F}_ i$ such that for each $i$ the support of $\mathcal{F}_ i$ is contained in a closed subscheme not containing $\eta $.

Proof. Proof of (1). We can write $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits _{i \in I} \mathcal{F}_ i$ with $\mathcal{F}_ i$ constructible abelian by Lemma 57.72.2. Choose $i \in I$. Since $\mathcal{F}|_\eta $ is zero by assumption, we see that there exists an $i'(i) \geq i$ such that $\mathcal{F}_ i|_\eta \to \mathcal{F}_{i'(i)}|_\eta $ is zero, see Lemma 57.70.9. Then $\mathcal{G}_ i = \mathop{\mathrm{Im}}(\mathcal{F}_ i \to \mathcal{F}_{i'(i)})$ is a constructible abelian sheaf (Lemma 57.70.6) whose stalk at $\eta $ is zero. Hence the support $E_ i$ of $\mathcal{G}_ i$ is a constructible subset of $X$ not containing $\eta $. Since $\eta $ is a generic point of an irreducible component of $X$, we see that $\eta \not\in Z_ i = \overline{E_ i}$ by Topology, Lemma 5.15.15. Define a new directed set $I'$ by using the set $I$ with ordering defined by the rule $i_1$ is bigger or equal to $i_2$ if and only if $i_1 \geq i'(i_2)$. Then the sheaves $\mathcal{G}_ i$ form a system over $I'$ with colimit $\mathcal{F}$ and the proof is complete.

The proof in case (2) is exactly the same and we omit it. $\square$


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