The Stacks project

59.89 Smooth base change

In this section we prove the smooth base change theorem.

Lemma 59.89.1. Let $K/k$ be an extension of fields. Let $X$ be a smooth affine curve over $k$ with a rational point $x \in X(k)$. Let $\mathcal{F}$ be an abelian sheaf on $\mathop{\mathrm{Spec}}(K)$ annihilated by an integer $n$ invertible in $k$. Let $q > 0$ and

\[ \xi \in H^ q(X_ K, (X_ K \to \mathop{\mathrm{Spec}}(K))^{-1}\mathcal{F}) \]

There exist

  1. finite extensions $K'/K$ and $k'/k$ with $k' \subset K'$,

  2. a finite étale Galois cover $Z \to X_{k'}$ with group $G$

such that the order of $G$ divides a power of $n$, such that $Z \to X_{k'}$ is split over $x_{k'}$, and such that $\xi $ dies in $H^ q(Z_{K'}, (Z_{K'} \to \mathop{\mathrm{Spec}}(K))^{-1}\mathcal{F})$.

Proof. For $q > 1$ we know that $\xi $ dies in $H^ q(X_{\overline{K}}, (X_{\overline{K}} \to \mathop{\mathrm{Spec}}(K))^{-1}\mathcal{F})$ (Theorem 59.83.10). By Lemma 59.51.5 we see that this means there is a finite extension $K'/K$ such that $\xi $ dies in $H^ q(X_{K'}, (X_{K'} \to \mathop{\mathrm{Spec}}(K))^{-1}\mathcal{F})$. Thus we can take $k' = k$ and $Z = X$ in this case.

Assume $q = 1$. Recall that $\mathcal{F}$ corresponds to a discrete module $M$ with continuous $\text{Gal}_ K$-action, see Lemma 59.59.1. Since $M$ is $n$-torsion, it is the uninon of finite $\text{Gal}_ K$-stable subgroups. Thus we reduce to the case where $M$ is a finite abelian group annihilated by $n$, see Lemma 59.51.4. After replacing $K$ by a finite extension we may assume that the action of $\text{Gal}_ K$ on $M$ is trivial. Thus we may assume $\mathcal{F} = \underline{M}$ is the constant sheaf with value a finite abelian group $M$ annihilated by $n$.

We can write $M$ as a direct sum of cyclic groups. Any two finite étale Galois coverings whose Galois groups have order invertible in $k$, can be dominated by a third one whose Galois group has order invertible in $k$ (Fundamental Groups, Section 58.7). Thus it suffices to prove the lemma when $M = \mathbf{Z}/d\mathbf{Z}$ where $d | n$.

Assume $M = \mathbf{Z}/d\mathbf{Z}$ where $d | n$. In this case $\overline{\xi } = \xi |_{X_{\overline{K}}}$ is an element of

\[ H^1(X_{\overline{k}}, \mathbf{Z}/d\mathbf{Z}) = H^1(X_{\overline{K}}, \mathbf{Z}/d\mathbf{Z}) \]

See Theorem 59.83.10. This group classifies $\mathbf{Z}/d\mathbf{Z}$-torsors, see Cohomology on Sites, Lemma 21.4.3. The torsor corresponding to $\overline{\xi }$ (viewed as a sheaf on $X_{\overline{k}, {\acute{e}tale}}$) in turn gives rise to a finite étale morphism $T \to X_{\overline{k}}$ endowed an action of $\mathbf{Z}/d\mathbf{Z}$ transitive on the fibre of $T$ over $x_{\overline{k}}$, see Lemma 59.64.4. Choose a connected component $T' \subset T$ (if $\overline{\xi }$ has order $d$, then $T$ is already connected). Then $T' \to X_{\overline{k}}$ is a finite étale Galois cover whose Galois group is a subgroup $G \subset \mathbf{Z}/d\mathbf{Z}$ (small detail omitted). Moreover the element $\overline{\xi }$ maps to zero under the map $H^1(X_{\overline{k}}, \mathbf{Z}/d\mathbf{Z}) \to H^1(T', \mathbf{Z}/d\mathbf{Z})$ as this is one of the defining properties of $T$.

Next, we use a limit argument to choose a finite extension $k'/k$ contained in $\overline{k}$ such that $T' \to X_{\overline{k}}$ descends to a finite étale Galois cover $Z \to X_{k'}$ with group $G$. See Limits, Lemmas 32.10.1, 32.8.3, and 32.8.10. After increasing $k'$ we may assume that $Z$ splits over $x_{k'}$. The image of $\xi $ in $H^1(Z_{\overline{K}}, \mathbf{Z}/d\mathbf{Z})$ is zero by construction. Thus by Lemma 59.51.5 we can find a finite subextension $\overline{K}/K'/K$ containing $k'$ such that $\xi $ dies in $H^1(Z_{K'}, \mathbf{Z}/d\mathbf{Z})$ and this finishes the proof. $\square$

Theorem 59.89.2 (Smooth base change). Consider a cartesian diagram of schemes

\[ \xymatrix{ X \ar[d]_ f & Y \ar[l]^ h \ar[d]^ e \\ S & T \ar[l]_ g } \]

where $f$ is smooth and $g$ quasi-compact and quasi-separated. Then

\[ f^{-1}R^ qg_*\mathcal{F} = R^ qh_*e^{-1}\mathcal{F} \]

for any $q$ and any abelian sheaf $\mathcal{F}$ on $T_{\acute{e}tale}$ all of whose stalks at geometric points are torsion of orders invertible on $S$.

First proof of smooth base change. This proof is very long but more direct (using less general theory) than the second proof given below.

The theorem is local on $X_{\acute{e}tale}$. More precisely, suppose we have $U \to X$ étale such that $U \to S$ factors as $U \to V \to S$ with $V \to S$ étale. Then we can consider the cartesian square

\[ \xymatrix{ U \ar[d]_{f'} & U \times _ X Y \ar[l]^{h'} \ar[d]^{e'} \\ V & V \times _ S T \ar[l]_{g'} } \]

and setting $\mathcal{F}' = \mathcal{F}|_{V \times _ S T}$ we have $f^{-1}R^ qg_*\mathcal{F}|_ U = (f')^{-1}R^ qg'_*\mathcal{F}'$ and $R^ qh_*e^{-1}\mathcal{F}|_ U = R^ qh'_*(e')^{-1}\mathcal{F}'$ (as follows from the compatibility of localization with morphisms of sites, see Sites, Lemma 7.28.2 and and Cohomology on Sites, Lemma 21.20.4). Thus it suffices to produce an étale covering of $X$ by $U \to X$ and factorizations $U \to V \to S$ as above such that the theorem holds for the diagram with $f'$, $h'$, $g'$, $e'$.

By the local structure of smooth morphisms, see Morphisms, Lemma 29.36.20, we may assume $X$ and $S$ are affine and $X \to S$ factors through an étale morphism $X \to \mathbf{A}^ d_ S$. If we have a tower of cartesian diagrams

\[ \xymatrix{ W \ar[d]_ i & Z \ar[l]^ j \ar[d]^ k \\ X \ar[d]_ f & Y \ar[l]^ h \ar[d]^ e \\ S & T \ar[l]_ g } \]

and the theorem holds for the bottom and top squares, then the theorem holds for the outer rectangle; this is formal. Writing $X \to S$ as the composition

\[ X \to \mathbf{A}^{d - 1}_ S \to \mathbf{A}^{d - 2}_ S \to \ldots \to \mathbf{A}^1_ S \to S \]

we conclude that it suffices to prove the theorem when $X$ and $S$ are affine and $X \to S$ has relative dimension $1$.

For every $n \geq 1$ invertible on $S$, let $\mathcal{F}[n]$ be the subsheaf of sections of $\mathcal{F}$ annihilated by $n$. Then $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits \mathcal{F}[n]$ by our assumption on the stalks of $\mathcal{F}$. The functors $e^{-1}$ and $f^{-1}$ commute with colimits as they are left adjoints. The functors $R^ qh_*$ and $R^ qg_*$ commute with filtered colimits by Lemma 59.51.7. Thus it suffices to prove the theorem for $\mathcal{F}[n]$. From now on we fix an integer $n$, we work with sheaves of $\mathbf{Z}/n\mathbf{Z}$-modules and we assume $S$ is a scheme over $\mathop{\mathrm{Spec}}(\mathbf{Z}[1/n])$.

Next, we reduce to the case where $T$ is affine. Since $g$ is quasi-compact and quasi-separate and $S$ is affine, the scheme $T$ is quasi-compact and quasi-separated. Thus we can use the induction principle of Cohomology of Schemes, Lemma 30.4.1. Hence it suffices to show that if $T = W \cup W'$ is an open covering and the theorem holds for the squares

\[ \xymatrix{ X \ar[d] & e^{-1}(W) \ar[l]^ i \ar[d] \\ S & W \ar[l]_ a } \quad \xymatrix{ X \ar[d] & e^{-1}(W') \ar[l]^ j \ar[d] \\ S & W' \ar[l]_ b } \quad \xymatrix{ X \ar[d] & e^{-1}(W \cap W') \ar[l]^-k \ar[d] \\ S & W \cap W' \ar[l]_ c } \]

then the theorem holds for the original diagram. To see this we consider the diagram

\[ \xymatrix{ f^{-1}R^{q - 1}c_*\mathcal{F}|_{W \cap W'} \ar[d]^{\cong } \ar[r] & f^{-1}R^ qg_*\mathcal{F} \ar[d] \ar[r] & f^{-1}R^ qa_*\mathcal{F}|_ W \oplus f^{-1}R^ qb_*\mathcal{F}|_{W'} \ar[d]_{\cong } \\ R^ qk_*e^{-1}\mathcal{F}|_{e^{-1}(W \cap W')} \ar[r] & R^ qh_*e^{-1}\mathcal{F} \ar[r] & R^ qi_*e^{-1}\mathcal{F}|_{e^{-1}(W)} \oplus R^ qj_*e^{-1}\mathcal{F}|_{e^{-1}(W')} } \]

whose rows are the long exact sequences of Lemma 59.50.2. Thus the $5$-lemma gives the desired conclusion.

Summarizing, we may assume $S$, $X$, $T$, and $Y$ affine, $\mathcal{F}$ is $n$ torsion, $X \to S$ is smooth of relative dimension $1$, and $S$ is a scheme over $\mathbf{Z}[1/n]$. We will prove the theorem by induction on $q$. The base case $q = 0$ is handled by Lemma 59.87.2. Assume $q > 0$ and the theorem holds for all smaller degrees. Choose a short exact sequence $0 \to \mathcal{F} \to \mathcal{I} \to \mathcal{Q} \to 0$ where $\mathcal{I}$ is an injective sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules. Consider the induced diagram

\[ \xymatrix{ f^{-1}R^{q - 1}g_*\mathcal{I} \ar[d]_{\cong } \ar[r] & f^{-1}R^{q - 1}g_*\mathcal{Q} \ar[d]_{\cong } \ar[r] & f^{-1}R^ qg_*\mathcal{F} \ar[d] \ar[r] & 0 \ar[d] \\ R^{q - 1}h_*e^{-1}\mathcal{I} \ar[r] & R^{q - 1}h_*e^{-1}\mathcal{Q} \ar[r] & R^ qh_*e^{-1}\mathcal{F} \ar[r] & R^ qh_*e^{-1}\mathcal{I} } \]

with exact rows. We have the zero in the right upper corner as $\mathcal{I}$ is injective. The left two vertical arrows are isomorphisms by induction hypothesis. Thus it suffices to prove that $R^ qh_*e^{-1}\mathcal{I} = 0$.

Write $S = \mathop{\mathrm{Spec}}(A)$ and $T = \mathop{\mathrm{Spec}}(B)$ and say the morphism $T \to S$ is given by the ring map $A \to B$. We can write $A \to B = \mathop{\mathrm{colim}}\nolimits _{i \in I} (A_ i \to B_ i)$ as a filtered colimit of maps of rings of finite type over $\mathbf{Z}[1/n]$ (see Algebra, Lemma 10.127.14). For $i \in I$ we set $S_ i = \mathop{\mathrm{Spec}}(A_ i)$ and $T_ i = \mathop{\mathrm{Spec}}(B_ i)$. For $i$ large enough we can find a smooth morphism $X_ i \to S_ i$ of relative dimension $1$ such that $X = X_ i \times _{S_ i} S$, see Limits, Lemmas 32.10.1, 32.8.9, and 32.18.3. Set $Y_ i = X_ i \times _{S_ i} T_ i$ to get squares

\[ \xymatrix{ X_ i \ar[d]_{f_ i} & Y_ i \ar[l]^{h_ i} \ar[d]^{e_ i} \\ S_ i & T_ i \ar[l]_{g_ i} } \]

Observe that $\mathcal{I}_ i = (T \to T_ i)_*\mathcal{I}$ is an injective sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules on $T_ i$, see Cohomology on Sites, Lemma 21.14.2. We have $\mathcal{I} = \mathop{\mathrm{colim}}\nolimits (T \to T_ i)^{-1}\mathcal{I}_ i$ by Lemma 59.51.9. Pulling back by $e$ we get $e^{-1}\mathcal{I} = \mathop{\mathrm{colim}}\nolimits (Y \to Y_ i)^{-1}e_ i^{-1}\mathcal{I}_ i$. By Lemma 59.51.8 applied to the system of morphisms $Y_ i \to X_ i$ with limit $Y \to X$ we have

\[ R^ qh_*e^{-1}\mathcal{I} = \mathop{\mathrm{colim}}\nolimits (X \to X_ i)^{-1} R^ qh_{i, *} e_ i^{-1}\mathcal{I}_ i \]

This reduces us to the case where $T$ and $S$ are affine of finite type over $\mathbf{Z}[1/n]$.

Summarizing, we have an integer $q \geq 1$ such that the theorem holds in degrees $< q$, the schemes $S$ and $T$ affine of finite type type over $\mathbf{Z}[1/n]$, we have $X \to S$ smooth of relative dimension $1$ with $X$ affine, and $\mathcal{I}$ is an injective sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules and we have to show that $R^ qh_*e^{-1}\mathcal{I} = 0$. We will do this by induction on $\dim (T)$.

The base case is $T = \emptyset $, i.e., $\dim (T) < 0$. If you don't like this, you can take as your base case the case $\dim (T) = 0$. In this case $T \to S$ is finite (in fact even $T \to \mathop{\mathrm{Spec}}(\mathbf{Z}[1/n])$ is finite as the target is Jacobson; details omitted), so $h$ is finite too and hence has vanishing higher direct images (see references below).

Assume $\dim (T) = d \geq 0$ and we know the result for all situations where $T$ has lower dimension. Pick $U$ affine and étale over $X$ and a section $\xi $ of $R^ qh_*q^{-1}\mathcal{I}$ over $U$. We have to show that $\xi $ is zero. Of course, we may replace $X$ by $U$ (and correspondingly $Y$ by $U \times _ X Y$) and assume $\xi \in H^0(X, R^ qh_*e^{-1}\mathcal{I})$. Moreover, since $R^ qh_*e^{-1}\mathcal{I}$ is a sheaf, it suffices to prove that $\xi $ is zero locally on $X$. Hence we may replace $X$ by the members of an étale covering. In particular, using Lemma 59.51.6 we may assume that $\xi $ is the image of an element $\tilde\xi \in H^ q(Y, e^{-1}\mathcal{I})$. In terms of $\tilde\xi $ our task is to show that $\tilde\xi $ dies in $H^ q(U_ i \times _ X Y, e^{-1}\mathcal{I})$ for some étale covering $\{ U_ i \to X\} $.

By More on Morphisms, Lemma 37.38.8 we may assume that $X \to S$ factors as $X \to V \to S$ where $V \to S$ is étale and $X \to V$ is a smooth morphism of affine schemes of relative dimension $1$, has a section, and has geometrically connected fibres. Observe that $\dim (V \times _ S T) \leq \dim (T) = d$ for example by More on Algebra, Lemma 15.44.2. Hence we may then replace $S$ by $V$ and $T$ by $V \times _ S T$ (exactly as in the discussion in the first paragraph of the proof). Thus we may assume $X \to S$ is smooth of relative dimension $1$, geometrically connected fibres, and has a section $\sigma : S \to X$.

Let $\pi : T' \to T$ be a finite surjective morphism. We will use below that $\dim (T') \leq \dim (T) = d$, see Algebra, Lemma 10.112.3. Choose an injective map $\pi ^{-1}\mathcal{I} \to \mathcal{I}'$ into an injective sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules. Then $\mathcal{I} \to \pi _*\mathcal{I}'$ is injective and hence has a splitting (as $\mathcal{I}$ is an injective sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules). Denote $\pi ' : Y' = Y \times _ T T' \to Y$ the base change of $\pi $ and $e' : Y' \to T'$ the base change of $e$. Picture

\[ \xymatrix{ X \ar[d]_ f & Y \ar[l]^ h \ar[d]^ e & Y' \ar[l]^{\pi '} \ar[d]^{e'} \\ S & T \ar[l]_ g & T' \ar[l]_\pi } \]

By Proposition 59.55.2 and Lemma 59.55.3 we have $R\pi '_*(e')^{-1}\mathcal{I}' = e^{-1}\pi _*\mathcal{I}'$. Thus by the Leray spectral sequence (Cohomology on Sites, Lemma 21.14.5) we have

\[ H^ q(Y', (e')^{-1}\mathcal{I}') = H^ q(Y, e^{-1}\pi _*\mathcal{I}') \supset H^ q(Y, e^{-1}\mathcal{I}) \]

and this remains true after base change by any $U \to X$ étale. Thus we may replace $T$ by $T'$, $\mathcal{I}$ by $\mathcal{I}'$ and $\tilde\xi $ by its image in $H^ q(Y', (e')^{-1}\mathcal{I}')$.

Suppose we have a factorization $T \to S' \to S$ where $\pi : S' \to S$ is finite. Setting $X' = S' \times _ S X$ we can consider the induced diagram

\[ \xymatrix{ X \ar[d]_ f & X' \ar[l]^{\pi '} \ar[d]^{f'} & Y \ar[l]^{h'} \ar[d]^ e \\ S & S' \ar[l]_\pi & T \ar[l]_ g } \]

Since $\pi '$ has vanishing higher direct images we see that $R^ qh_*e^{-1}\mathcal{I} = \pi '_*R^ qh'_*e^{-1}\mathcal{I}$ by the Leray spectral sequence. Hence $H^0(X, R^ qh_*e^{-1}\mathcal{I}) = H^0(X', R^ qh'_*e^{-1}\mathcal{I})$. Thus $\xi $ is zero if and only if the corresponding section of $R^ qh'_*e^{-1}\mathcal{I}$ is zero1. Thus we may replace $S$ by $S'$ and $X$ by $X'$. Observe that $\sigma : S \to X$ base changes to $\sigma ' : S' \to X'$ and hence after this replacement it is still true that $X \to S$ has a section $\sigma $ and geometrically connected fibres.

We will use that $S$ and $T$ are Nagata schemes, see Algebra, Proposition 10.162.16 which will guarantee that various normalizations are finite, see Morphisms, Lemmas 29.53.15 and 29.54.10. In particular, we may first replace $T$ by its normalization and then replace $S$ by the normalization of $S$ in $T$. Then $T \to S$ is a disjoint union of dominant morphisms of integral normal schemes, see Morphisms, Lemma 29.53.13. Clearly we may argue one connnected component at a time, hence we may assume $T \to S$ is a dominant morphism of integral normal schemes.

Let $s \in S$ and $t \in T$ be the generic points. By Lemma 59.89.1 there exist finite field extensions $K/\kappa (t)$ and $k/\kappa (s)$ such that $k$ is contained in $K$ and a finite étale Galois covering $Z \to X_ k$ with Galois group $G$ of order dividing a power of $n$ split over $\sigma (\mathop{\mathrm{Spec}}(k))$ such that $\tilde\xi $ maps to zero in $H^ q(Z_ K, e^{-1}\mathcal{I}|_{Z_ K})$. Let $T' \to T$ be the normalization of $T$ in $\mathop{\mathrm{Spec}}(K)$ and let $S' \to S$ be the normalization of $S$ in $\mathop{\mathrm{Spec}}(k)$. Then we obtain a commutative diagram

\[ \xymatrix{ S' \ar[d] & T' \ar[l] \ar[d] \\ S & T \ar[l] } \]

whose vertical arrows are finite. By the arguments given above we may and do replace $S$ and $T$ by $S'$ and $T'$ (and correspondingly $X$ by $X \times _ S S'$ and $Y$ by $Y \times _ T T'$). After this replacement we conclude we have a finite étale Galois covering $Z \to X_ s$ of the generic fibre of $X \to S$ with Galois group $G$ of order dividing a power of $n$ split over $\sigma (s)$ such that $\tilde\xi $ maps to zero in $H^ q(Z_ t, (Z_ t \to Y)^{-1}e^{-1}\mathcal{I})$. Here $Z_ t = Z \times _ S t = Z \times _ s t = Z \times _{X_ s} Y_ t$. Since $n$ is invertible on $S$, by Fundamental Groups, Lemma 58.31.7 we can find a finite étale morphism $U \to X$ whose restriction to $X_ s$ is $Z$.

At this point we replace $X$ by $U$ and $Y$ by $U \times _ X Y$. After this replacement it may no longer be the case that the fibres of $X \to S$ are geometrically connected (there still is a section but we won't use this), but what we gain is that after this replacement $\tilde\xi $ maps to zero in $H^ q(Y_ t, e^{-1}\mathcal{I})$, i.e., $\tilde\xi $ restricts to zero on the generic fibre of $Y \to T$.

Recall that $t$ is the spectrum of the function field of $T$, i.e., as a scheme $t$ is the limit of the nonempty affine open subschemes of $T$. By Lemma 59.51.5 we conclude there exists a nonempty open subscheme $V \subset T$ such that $\tilde\xi $ maps to zero in $H^ q(Y \times _ T V, e^{-1}\mathcal{I}|_{Y \times _ T V})$.

Denote $Z = T \setminus V$. Consider the diagram

\[ \xymatrix{ Y \times _ T Z \ar[d]_{e_ Z} \ar[r]_{i'} & Y \ar[d]_ e & Y \times _ T V \ar[l]^{j'} \ar[d]^{e_ V} \\ Z \ar[r]^ i & T & V \ar[l]_ j } \]

Choose an injection $i^{-1}\mathcal{I} \to \mathcal{I}'$ into an injective sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules on $Z$. Looking at stalks we see that the map

\[ \mathcal{I} \to j_*\mathcal{I}|_ V \oplus i_*\mathcal{I}' \]

is injective and hence splits as $\mathcal{I}$ is an injective sheaf of $\mathbf{Z}/n\mathbf{Z}$-modules. Thus it suffices to show that $\tilde\xi $ maps to zero in

\[ H^ q(Y, e^{-1}j_*\mathcal{I}|_ V) \oplus H^ q(Y, e^{-1}i_*\mathcal{I}') \]

at least after replacing $X$ by the members of an étale covering. Observe that

\[ e^{-1}j_*\mathcal{I}|_ V = j'_*e_ V^{-1}\mathcal{I}|_ V,\quad e^{-1}i_*\mathcal{I}' = i'_*e_ Z^{-1}\mathcal{I}' \]

By induction hypothesis on $q$ we see that

\[ R^ aj'_*e_ V^{-1}\mathcal{I}|_ V = 0, \quad a = 1, \ldots , q - 1 \]

By the Leray spectral sequence for $j'$ and the vanishing above it follows that

\[ H^ q(Y, j'_*(e_ V^{-1}\mathcal{I}|_ V)) \longrightarrow H^ q(Y \times _ T V, e_ V^{-1}\mathcal{I}_ V) = H^ q(Y \times _ T V, e^{-1}\mathcal{I}|_{Y \times _ T V}) \]

is injective. Thus the vanishing of the image of $\tilde\xi $ in the first summand above because we know $\tilde\xi $ vanishes in $H^ q(Y \times _ T V, e^{-1}\mathcal{I}|_{Y \times _ T V})$. Since $\dim (Z) < \dim (T) = d$ by induction the image of $\tilde\xi $ in the second summand

\[ H^ q(Y, e^{-1}i_*\mathcal{I}') = H^ q(Y, i'_*e_ Z^{-1}\mathcal{I}') = H^ q(Y \times _ T Z, e_ Z^{-1}\mathcal{I}') \]

dies after replacing $X$ by the members of a suitable étale covering. This finishes the proof of the smooth base change theorem. $\square$

Second proof of smooth base change. This proof is the same as the longer first proof; it is shorter only in that we have split out the arguments used in a number of lemmas.

The case of $q = 0$ is Lemma 59.87.2. Thus we may assume $q > 0$ and the result is true for all smaller degrees.

For every $n \geq 1$ invertible on $S$, let $\mathcal{F}[n]$ be the subsheaf of sections of $\mathcal{F}$ annihilated by $n$. Then $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits \mathcal{F}[n]$ by our assumption on the stalks of $\mathcal{F}$. The functors $e^{-1}$ and $f^{-1}$ commute with colimits as they are left adjoints. The functors $R^ qh_*$ and $R^ qg_*$ commute with filtered colimits by Lemma 59.51.7. Thus it suffices to prove the theorem for $\mathcal{F}[n]$. From now on we fix an integer $n$ invertible on $S$ and we work with sheaves of $\mathbf{Z}/n\mathbf{Z}$-modules.

By Lemma 59.86.1 the question is étale local on $X$ and $S$. By the local structure of smooth morphisms, see Morphisms, Lemma 29.36.20, we may assume $X$ and $S$ are affine and $X \to S$ factors through an étale morphism $X \to \mathbf{A}^ d_ S$. Writing $X \to S$ as the composition

\[ X \to \mathbf{A}^{d - 1}_ S \to \mathbf{A}^{d - 2}_ S \to \ldots \to \mathbf{A}^1_ S \to S \]

we conclude from Lemma 59.86.2 that it suffices to prove the theorem when $X$ and $S$ are affine and $X \to S$ has relative dimension $1$.

By Lemma 59.88.7 it suffices to show that $R^ qh_*\mathbf{Z}/d\mathbf{Z} = 0$ for $d | n$ whenever we have a cartesian diagram

\[ \xymatrix{ X \ar[d] & Y \ar[d] \ar[l]^ h \\ S & \mathop{\mathrm{Spec}}(K) \ar[l] } \]

where $X \to S$ is affine and smooth of relative dimension $1$, $S$ is the spectrum of a normal domain $A$ with algebraically closed fraction field $L$, and $K/L$ is an extension of algebraically closed fields.

Recall that $R^ qh_*\mathbf{Z}/d\mathbf{Z}$ is the sheaf associated to the presheaf

\[ U \longmapsto H^ q(U \times _ X Y, \mathbf{Z}/d\mathbf{Z}) = H^ q(U \times _ S \mathop{\mathrm{Spec}}(K), \mathbf{Z}/d\mathbf{Z}) \]

on $X_{\acute{e}tale}$ (Lemma 59.51.6). Thus it suffices to show: given $U$ and $\xi \in H^ q(U \times _ S \mathop{\mathrm{Spec}}(K), \mathbf{Z}/d\mathbf{Z})$ there exists an étale covering $\{ U_ i \to U\} $ such that $\xi $ dies in $H^ q(U_ i \times _ S \mathop{\mathrm{Spec}}(K), \mathbf{Z}/d\mathbf{Z})$.

Of course we may take $U$ affine. Then $U \times _ S \mathop{\mathrm{Spec}}(K)$ is a (smooth) affine curve over $K$ and hence we have vanishing for $q > 1$ by Theorem 59.83.10.

Final case: $q = 1$. We may replace $U$ by the members of an étale covering as in More on Morphisms, Lemma 37.38.8. Then $U \to S$ factors as $U \to V \to S$ where $U \to V$ has geometrically connected fibres, $U$, $V$ are affine, $V \to S$ is étale, and there is a section $\sigma : V \to U$. By Lemma 59.80.4 we see that $V$ is isomorphic to a (finite) disjoint union of (affine) open subschemes of $S$. Clearly we may replace $S$ by one of these and $X$ by the corresponding component of $U$. Thus we may assume $X \to S$ has geometrically connected fibres, has a section $\sigma $, and $\xi \in H^1(Y, \mathbf{Z}/d\mathbf{Z})$. Since $K$ and $L$ are algebraically closed we have

\[ H^1(X_ L, \mathbf{Z}/d\mathbf{Z}) = H^1(Y, \mathbf{Z}/d\mathbf{Z}) \]

See Lemma 59.83.12. Thus there is a finite étale Galois covering $Z \to X_ L$ with Galois group $G \subset \mathbf{Z}/d\mathbf{Z}$ which annihilates $\xi $. You can either see this by looking at the statement or proof of Lemma 59.89.1 or by using directly that $\xi $ corresponds to a $\mathbf{Z}/d\mathbf{Z}$-torsor over $X_ L$. Finally, by Fundamental Groups, Lemma 58.31.8 we find a (necessarily surjective) finite étale morphism $X' \to X$ whose restriction to $X_ L$ is $Z \to X_ L$. Since $\xi $ dies in $X'_ K$ this finishes the proof. $\square$

The following immediate consquence of the smooth base change theorem is what is often used in practice.

Lemma 59.89.3. Let $S$ be a scheme. Let $S' = \mathop{\mathrm{lim}}\nolimits S_ i$ be a directed inverse limit of schemes $S_ i$ smooth over $S$ with affine transition morphisms. Let $f : X \to S$ be quasi-compact and quasi-separated and form the fibre square

\[ \xymatrix{ X' \ar[d]_{f'} \ar[r]_{g'} & X \ar[d]^ f \\ S' \ar[r]^ g & S } \]

Then

\[ g^{-1}Rf_*E = R(f')_*(g')^{-1}E \]

for any $E \in D^+(X_{\acute{e}tale})$ whose cohomology sheaves $H^ q(E)$ have stalks which are torsion of orders invertible on $S$.

Proof. Consider the spectral sequences

\[ E_2^{p, q} = R^ pf_*H^ q(E) \quad \text{and}\quad {E'}_2^{p, q} = R^ pf'_*H^ q((g')^{-1}E) = R^ pf'_*(g')^{-1}H^ q(E) \]

converging to $R^ nf_*E$ and $R^ nf'_*(g')^{-1}E$. These spectral sequences are constructed in Derived Categories, Lemma 13.21.3. Combining the smooth base change theorem (Theorem 59.89.2) with Lemma 59.86.3 we see that

\[ g^{-1}R^ pf_*H^ q(E) = R^ p(f')_*(g')^{-1}H^ q(E) \]

Combining all of the above we get the lemma. $\square$

[1] This step can also be seen another way. Namely, we have to show that there is an étale covering $\{ U_ i \to X\} $ such that $\tilde\xi $ dies in $H^ q(U_ i \times _ X Y, e^{-1}\mathcal{I})$. However, if we prove there is an étale covering $\{ U'_ j \to X'\} $ such that $\tilde\xi $ dies in $H^ q(U'_ i \times _{X'} Y, e^{-1}\mathcal{I})$, then by property (B) for $X' \to X$ (Lemma 59.43.3) there exists an étale covering $\{ U_ i \to X\} $ such that $U_ i \times _ X X'$ is a disjoint union of schemes over $X'$ each of which factors through $U'_ j$ for some $j$. Thus we see that $\tilde\xi $ dies in $H^ q(U_ i \times _ X Y, e^{-1}\mathcal{I})$ as desired.

Comments (3)

Comment #5256 by Anonymous on

There is a diagram which is not compiling

Comment #5901 by Harry Gindi on

As anonymous said, there is a broken diagram.


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