The Stacks project

34.29 Properties of morphisms étale local on source-and-target

Let $\mathcal{P}$ be a property of morphisms of schemes. There is an intuitive meaning to the phrase “$\mathcal{P}$ is étale local on the source and target”. However, it turns out that this notion is not the same as asking $\mathcal{P}$ to be both étale local on the source and étale local on the target. Before we discuss this further we give two silly examples.

Example 34.29.1. Consider the property $\mathcal{P}$ of morphisms of schemes defined by the rule $\mathcal{P}(X \to Y) = $“$Y$ is locally Noetherian”. The reader can verify that this is étale local on the source and étale local on the target (omitted, see Lemma 34.13.1). But it is not true that if $f : X \to Y$ has $\mathcal{P}$ and $g : Y \to Z$ is étale, then $g \circ f$ has $\mathcal{P}$. Namely, $f$ could be the identity on $Y$ and $g$ could be an open immersion of a locally Noetherian scheme $Y$ into a non locally Noetherian scheme $Z$.

The following example is in some sense worse.

Example 34.29.2. Consider the property $\mathcal{P}$ of morphisms of schemes defined by the rule $\mathcal{P}(f : X \to Y) = $“for every $y \in Y$ which is a specialization of some $f(x)$, $x \in X$ the local ring $\mathcal{O}_{Y, y}$ is Noetherian”. Let us verify that this is étale local on the source and étale local on the target. We will freely use Schemes, Lemma 25.13.2.

Local on the target: Let $\{ g_ i : Y_ i \to Y\} $ be an étale covering. Let $f_ i : X_ i \to Y_ i$ be the base change of $f$, and denote $h_ i : X_ i \to X$ the projection. Assume $\mathcal{P}(f)$. Let $f(x_ i) \leadsto y_ i$ be a specialization. Then $f(h_ i(x_ i)) \leadsto g_ i(y_ i)$ so $\mathcal{P}(f)$ implies $\mathcal{O}_{Y, g_ i(y_ i)}$ is Noetherian. Also $\mathcal{O}_{Y, g_ i(y_ i)} \to \mathcal{O}_{Y_ i, y_ i}$ is a localization of an étale ring map. Hence $\mathcal{O}_{Y_ i, y_ i}$ is Noetherian by Algebra, Lemma 10.30.1. Conversely, assume $\mathcal{P}(f_ i)$ for all $i$. Let $f(x) \leadsto y$ be a specialization. Choose an $i$ and $y_ i \in Y_ i$ mapping to $y$. Since $x$ can be viewed as a point of $\mathop{\mathrm{Spec}}(\mathcal{O}_{Y, y}) \times _ Y X$ and $\mathcal{O}_{Y, y} \to \mathcal{O}_{Y_ i, y_ i}$ is faithfully flat, there exists a point $x_ i \in \mathop{\mathrm{Spec}}(\mathcal{O}_{Y_ i, y_ i}) \times _ Y X$ mapping to $x$. Then $x_ i \in X_ i$, and $f_ i(x_ i)$ specializes to $y_ i$. Thus we see that $\mathcal{O}_{Y_ i, y_ i}$ is Noetherian by $\mathcal{P}(f_ i)$ which implies that $\mathcal{O}_{Y, y}$ is Noetherian by Algebra, Lemma 10.158.1.

Local on the source: Let $\{ h_ i : X_ i \to X\} $ be an étale covering. Let $f_ i : X_ i \to Y$ be the composition $f \circ h_ i$. Assume $\mathcal{P}(f)$. Let $f(x_ i) \leadsto y$ be a specialization. Then $f(h_ i(x_ i)) \leadsto y$ so $\mathcal{P}(f)$ implies $\mathcal{O}_{Y, y}$ is Noetherian. Thus $\mathcal{P}(f_ i)$ holds. Conversely, assume $\mathcal{P}(f_ i)$ for all $i$. Let $f(x) \leadsto y$ be a specialization. Choose an $i$ and $x_ i \in X_ i$ mapping to $x$. Then $y$ is a specialization of $f_ i(x_ i) = f(x)$. Hence $\mathcal{P}(f_ i)$ implies $\mathcal{O}_{Y, y}$ is Noetherian as desired.

We claim that there exists a commutative diagram

\[ \xymatrix{ U \ar[d]_ a \ar[r]_ h & V \ar[d]^ b \\ X \ar[r]^ f & Y } \]

with surjective étale vertical arrows, such that $h$ has $\mathcal{P}$ and $f$ does not have $\mathcal{P}$. Namely, let

\[ Y = \mathop{\mathrm{Spec}}\Big( \mathbf{C}[x_ n; n \in \mathbf{Z}]/(x_ n x_ m; n \not= m) \Big) \]

and let $X \subset Y$ be the open subscheme which is the complement of the point all of whose coordinates $x_ n = 0$. Let $U = X$, let $V = X \amalg Y$, let $a, b$ the obvious map, and let $h : U \to V$ be the inclusion of $U = X$ into the first summand of $V$. The claim above holds because $U$ is locally Noetherian, but $Y$ is not.

What should be the correct notion of a property which is étale local on the source-and-target? We think that, by analogy with Morphisms, Definition 28.13.1 it should be the following.

Definition 34.29.3. Let $\mathcal{P}$ be a property of morphisms of schemes. We say $\mathcal{P}$ is étale local on source-and-target if

  1. (stable under precomposing with étale maps) if $f : X \to Y$ is étale and $g : Y \to Z$ has $\mathcal{P}$, then $g \circ f$ has $\mathcal{P}$,

  2. (stable under étale base change) if $f : X \to Y$ has $\mathcal{P}$ and $Y' \to Y$ is étale, then the base change $f' : Y' \times _ Y X \to Y'$ has $\mathcal{P}$, and

  3. (locality) given a morphism $f : X \to Y$ the following are equivalent

    1. $f$ has $\mathcal{P}$,

    2. for every $x \in X$ there exists a commutative diagram

      \[ \xymatrix{ U \ar[d]_ a \ar[r]_ h & V \ar[d]^ b \\ X \ar[r]^ f & Y } \]

      with étale vertical arrows and $u \in U$ with $a(u) = x$ such that $h$ has $\mathcal{P}$.

It turns out this definition excludes the behavior seen in Examples 34.29.1 and 34.29.2. We will compare this to the definition in the paper [DM] by Deligne and Mumford in Remark 34.29.8. Moreover, a property which is étale local on the source-and-target is étale local on the source and étale local on the target. Finally, the converse is almost true as we will see in Lemma 34.29.6.

Lemma 34.29.4. Let $\mathcal{P}$ be a property of morphisms of schemes which is étale local on source-and-target. Then

  1. $\mathcal{P}$ is étale local on the source,

  2. $\mathcal{P}$ is étale local on the target,

  3. $\mathcal{P}$ is stable under postcomposing with étale morphisms: if $f : X \to Y$ has $\mathcal{P}$ and $g : Y \to Z$ is étale, then $g \circ f$ has $\mathcal{P}$, and

  4. $\mathcal{P}$ has a permanence property: given $f : X \to Y$ and $g : Y \to Z$ étale such that $g \circ f$ has $\mathcal{P}$, then $f$ has $\mathcal{P}$.

Proof. We write everything out completely.

Proof of (1). Let $f : X \to Y$ be a morphism of schemes. Let $\{ X_ i \to X\} _{i \in I}$ be an étale covering of $X$. If each composition $h_ i : X_ i \to Y$ has $\mathcal{P}$, then for each $x \in X$ we can find an $i \in I$ and a point $x_ i \in X_ i$ mapping to $x$. Then $(X_ i, x_ i) \to (X, x)$ is an étale morphism of germs, and $\text{id}_ Y : Y \to Y$ is an étale morphism, and $h_ i$ is as in part (3) of Definition 34.29.3. Thus we see that $f$ has $\mathcal{P}$. Conversely, if $f$ has $\mathcal{P}$ then each $X_ i \to Y$ has $\mathcal{P}$ by Definition 34.29.3 part (1).

Proof of (2). Let $f : X \to Y$ be a morphism of schemes. Let $\{ Y_ i \to Y\} _{i \in I}$ be an étale covering of $Y$. Write $X_ i = Y_ i \times _ Y X$ and $h_ i : X_ i \to Y_ i$ for the base change of $f$. If each $h_ i : X_ i \to Y_ i$ has $\mathcal{P}$, then for each $x \in X$ we pick an $i \in I$ and a point $x_ i \in X_ i$ mapping to $x$. Then $(X_ i, x_ i) \to (X, x)$ is an étale morphism of germs, $Y_ i \to Y$ is étale, and $h_ i$ is as in part (3) of Definition 34.29.3. Thus we see that $f$ has $\mathcal{P}$. Conversely, if $f$ has $\mathcal{P}$, then each $X_ i \to Y_ i$ has $\mathcal{P}$ by Definition 34.29.3 part (2).

Proof of (3). Assume $f : X \to Y$ has $\mathcal{P}$ and $g : Y \to Z$ is étale. For every $x \in X$ we can think of $(X, x) \to (X, x)$ as an étale morphism of germs, $Y \to Z$ is an étale morphism, and $h = f$ is as in part (3) of Definition 34.29.3. Thus we see that $g \circ f$ has $\mathcal{P}$.

Proof of (4). Let $f : X \to Y$ be a morphism and $g : Y \to Z$ étale such that $g \circ f$ has $\mathcal{P}$. Then by Definition 34.29.3 part (2) we see that $\text{pr}_ Y : Y \times _ Z X \to Y$ has $\mathcal{P}$. But the morphism $(f, 1) : X \to Y \times _ Z X$ is étale as a section to the étale projection $\text{pr}_ X : Y \times _ Z X \to X$, see Morphisms, Lemma 28.34.18. Hence $f = \text{pr}_ Y \circ (f, 1)$ has $\mathcal{P}$ by Definition 34.29.3 part (1). $\square$

The following lemma is the analogue of Morphisms, Lemma 28.13.4.

Lemma 34.29.5. Let $\mathcal{P}$ be a property of morphisms of schemes which is étale local on source-and-target. Let $f : X \to Y$ be a morphism of schemes. The following are equivalent:

  1. $f$ has property $\mathcal{P}$,

  2. for every $x \in X$ there exists an étale morphism of germs $a : (U, u) \to (X, x)$, an étale morphism $b : V \to Y$, and a morphism $h : U \to V$ such that $f \circ a = b \circ h$ and $h$ has $\mathcal{P}$,

  3. for any commutative diagram

    \[ \xymatrix{ U \ar[d]_ a \ar[r]_ h & V \ar[d]^ b \\ X \ar[r]^ f & Y } \]

    with $a$, $b$ étale the morphism $h$ has $\mathcal{P}$,

  4. for some diagram as in (c) with $a : U \to X$ surjective $h$ has $\mathcal{P}$,

  5. there exists an étale covering $\{ Y_ i \to Y\} _{i \in I}$ such that each base change $Y_ i \times _ Y X \to Y_ i$ has $\mathcal{P}$,

  6. there exists an étale covering $\{ X_ i \to X\} _{i \in I}$ such that each composition $X_ i \to Y$ has $\mathcal{P}$,

  7. there exists an étale covering $\{ Y_ i \to Y\} _{i \in I}$ and for each $i \in I$ an étale covering $\{ X_{ij} \to Y_ i \times _ Y X\} _{j \in J_ i}$ such that each morphism $X_{ij} \to Y_ i$ has $\mathcal{P}$.

Proof. The equivalence of (a) and (b) is part of Definition 34.29.3. The equivalence of (a) and (e) is Lemma 34.29.4 part (2). The equivalence of (a) and (f) is Lemma 34.29.4 part (1). As (a) is now equivalent to (e) and (f) it follows that (a) equivalent to (g).

It is clear that (c) implies (a). If (a) holds, then for any diagram as in (c) the morphism $f \circ a$ has $\mathcal{P}$ by Definition 34.29.3 part (1), whereupon $h$ has $\mathcal{P}$ by Lemma 34.29.4 part (4). Thus (a) and (c) are equivalent. It is clear that (c) implies (d). To see that (d) implies (a) assume we have a diagram as in (c) with $a : U \to X$ surjective and $h$ having $\mathcal{P}$. Then $b \circ h$ has $\mathcal{P}$ by Lemma 34.29.4 part (3). Since $\{ a : U \to X\} $ is an étale covering we conclude that $f$ has $\mathcal{P}$ by Lemma 34.29.4 part (1). $\square$

It seems that the result of the following lemma is not a formality, i.e., it actually uses something about the geometry of étale morphisms.

Lemma 34.29.6. Let $\mathcal{P}$ be a property of morphisms of schemes. Assume

  1. $\mathcal{P}$ is étale local on the source,

  2. $\mathcal{P}$ is étale local on the target, and

  3. $\mathcal{P}$ is stable under postcomposing with open immersions: if $f : X \to Y$ has $\mathcal{P}$ and $Y \subset Z$ is an open subscheme then $X \to Z$ has $\mathcal{P}$.

Then $\mathcal{P}$ is étale local on the source-and-target.

Proof. Let $\mathcal{P}$ be a property of morphisms of schemes which satisfies conditions (1), (2) and (3) of the lemma. By Lemma 34.23.2 we see that $\mathcal{P}$ is stable under precomposing with étale morphisms. By Lemma 34.19.2 we see that $\mathcal{P}$ is stable under étale base change. Hence it suffices to prove part (3) of Definition 34.29.3 holds.

More precisely, suppose that $f : X \to Y$ is a morphism of schemes which satisfies Definition 34.29.3 part (3)(b). In other words, for every $x \in X$ there exists an étale morphism $a_ x : U_ x \to X$, a point $u_ x \in U_ x$ mapping to $x$, an étale morphism $b_ x : V_ x \to Y$, and a morphism $h_ x : U_ x \to V_ x$ such that $f \circ a_ x = b_ x \circ h_ x$ and $h_ x$ has $\mathcal{P}$. The proof of the lemma is complete once we show that $f$ has $\mathcal{P}$. Set $U = \coprod U_ x$, $a = \coprod a_ x$, $V = \coprod V_ x$, $b = \coprod b_ x$, and $h = \coprod h_ x$. We obtain a commutative diagram

\[ \xymatrix{ U \ar[d]_ a \ar[r]_ h & V \ar[d]^ b \\ X \ar[r]^ f & Y } \]

with $a$, $b$ étale, $a$ surjective. Note that $h$ has $\mathcal{P}$ as each $h_ x$ does and $\mathcal{P}$ is étale local on the target. Because $a$ is surjective and $\mathcal{P}$ is étale local on the source, it suffices to prove that $b \circ h$ has $\mathcal{P}$. This reduces the lemma to proving that $\mathcal{P}$ is stable under postcomposing with an étale morphism.

During the rest of the proof we let $f : X \to Y$ be a morphism with property $\mathcal{P}$ and $g : Y \to Z$ is an étale morphism. Consider the following statements:

  1. With no additional assumptions $g \circ f$ has property $\mathcal{P}$.

  2. Whenever $Z$ is affine $g \circ f$ has property $\mathcal{P}$.

  3. Whenever $X$ and $Z$ are affine $g \circ f$ has property $\mathcal{P}$.

  4. Whenever $X$, $Y$, and $Z$ are affine $g \circ f$ has property $\mathcal{P}$.

Once we have proved (-) the proof of the lemma will be complete.

Claim 1: (AAA) $\Rightarrow $ (AA). Namely, let $f : X \to Y$, $g : Y \to Z$ be as above with $X$, $Z$ affine. As $X$ is affine hence quasi-compact we can find finitely many affine open $Y_ i \subset Y$, $i = 1, \ldots , n$ such that $X = \bigcup _{i = 1, \ldots , n} f^{-1}(Y_ i)$. Set $X_ i = f^{-1}(Y_ i)$. By Lemma 34.19.2 each of the morphisms $X_ i \to Y_ i$ has $\mathcal{P}$. Hence $\coprod _{i = 1, \ldots , n} X_ i \to \coprod _{i = 1, \ldots , n} Y_ i$ has $\mathcal{P}$ as $\mathcal{P}$ is étale local on the target. By (AAA) applied to $\coprod _{i = 1, \ldots , n} X_ i \to \coprod _{i = 1, \ldots , n} Y_ i$ and the étale morphism $\coprod _{i = 1, \ldots , n} Y_ i \to Z$ we see that $\coprod _{i = 1, \ldots , n} X_ i \to Z$ has $\mathcal{P}$. Now $\{ \coprod _{i = 1, \ldots , n} X_ i \to X\} $ is an étale covering, hence as $\mathcal{P}$ is étale local on the source we conclude that $X \to Z$ has $\mathcal{P}$ as desired.

Claim 2: (AAA) $\Rightarrow $ (A). Namely, let $f : X \to Y$, $g : Y \to Z$ be as above with $Z$ affine. Choose an affine open covering $X = \bigcup X_ i$. As $\mathcal{P}$ is étale local on the source we see that each $f|_{X_ i} : X_ i \to Y$ has $\mathcal{P}$. By (AA), which follows from (AAA) according to Claim 1, we see that $X_ i \to Z$ has $\mathcal{P}$ for each $i$. Since $\{ X_ i \to X\} $ is an étale covering and $\mathcal{P}$ is étale local on the source we conclude that $X \to Z$ has $\mathcal{P}$.

Claim 3: (AAA) $\Rightarrow $ (-). Namely, let $f : X \to Y$, $g : Y \to Z$ be as above. Choose an affine open covering $Z = \bigcup Z_ i$. Set $Y_ i = g^{-1}(Z_ i)$ and $X_ i = f^{-1}(Y_ i)$. By Lemma 34.19.2 each of the morphisms $X_ i \to Y_ i$ has $\mathcal{P}$. By (A), which follows from (AAA) according to Claim 2, we see that $X_ i \to Z_ i$ has $\mathcal{P}$ for each $i$. Since $\mathcal{P}$ is local on the target and $X_ i = (g \circ f)^{-1}(Z_ i)$ we conclude that $X \to Z$ has $\mathcal{P}$.

Thus to prove the lemma it suffices to prove (AAA). Let $f : X \to Y$ and $g : Y \to Z$ be as above $X, Y, Z$ affine. Note that an étale morphism of affines has universally bounded fibres, see Morphisms, Lemma 28.34.6 and Lemma 28.54.10. Hence we can do induction on the integer $n$ bounding the degree of the fibres of $Y \to Z$. See Morphisms, Lemma 28.54.9 for a description of this integer in the case of an étale morphism. If $n = 1$, then $Y \to Z$ is an open immersion, see Lemma 34.22.2, and the result follows from assumption (3) of the lemma. Assume $n > 1$.

Consider the following commutative diagram

\[ \xymatrix{ X \times _ Z Y \ar[d] \ar[r]_{f_ Y} & Y \times _ Z Y \ar[d] \ar[r]_-{\text{pr}} & Y \ar[d] \\ X \ar[r]^ f & Y \ar[r]^ g & Z } \]

Note that we have a decomposition into open and closed subschemes $Y \times _ Z Y = \Delta _{Y/Z}(Y) \amalg Y'$, see Morphisms, Lemma 28.33.13. As a base change the degrees of the fibres of the second projection $\text{pr} : Y \times _ Z Y \to Y$ are bounded by $n$, see Morphisms, Lemma 28.54.6. On the other hand, $\text{pr}|_{\Delta (Y)} : \Delta (Y) \to Y$ is an isomorphism and every fibre has exactly one point. Thus, on applying Morphisms, Lemma 28.54.9 we conclude the degrees of the fibres of the restriction $\text{pr}|_{Y'} : Y' \to Y$ are bounded by $n - 1$. Set $X' = f_ Y^{-1}(Y')$. Picture

\[ \xymatrix{ X \amalg X' \ar@{=}[d] \ar[r]_-{f \amalg f'} & \Delta (Y) \amalg Y' \ar@{=}[d] \ar[r] & Y \ar@{=}[d] \\ X \times _ Z Y \ar[r]^{f_ Y} & Y \times _ Z Y \ar[r]^-{\text{pr}} & Y } \]

As $\mathcal{P}$ is étale local on the target and hence stable under étale base change (see Lemma 34.19.2) we see that $f_ Y$ has $\mathcal{P}$. Hence, as $\mathcal{P}$ is étale local on the source, $f' = f_ Y|_{X'}$ has $\mathcal{P}$. By induction hypothesis we see that $X' \to Y$ has $\mathcal{P}$. As $\mathcal{P}$ is local on the source, and $\{ X \to X \times _ Z Y, X' \to X \times _ Y Z\} $ is an étale covering, we conclude that $\text{pr} \circ f_ Y$ has $\mathcal{P}$. Note that $g \circ f$ can be viewed as a morphism $g \circ f : X \to g(Y)$. As $\text{pr} \circ f_ Y$ is the pullback of $g \circ f : X \to g(Y)$ via the étale covering $\{ Y \to g(Y)\} $, and as $\mathcal{P}$ is étale local on the target, we conclude that $g \circ f : X \to g(Y)$ has property $\mathcal{P}$. Finally, applying assumption (3) of the lemma once more we conclude that $g \circ f : X \to Z$ has property $\mathcal{P}$. $\square$

Remark 34.29.7. Using Lemma 34.29.6 and the work done in the earlier sections of this chapter it is easy to make a list of types of morphisms which are étale local on the source-and-target. In each case we list the lemma which implies the property is étale local on the source and the lemma which implies the property is étale local on the target. In each case the third assumption of Lemma 34.29.6 is trivial to check, and we omit it. Here is the list:

  1. flat, see Lemmas 34.24.1 and 34.20.15,

  2. locally of finite presentation, see Lemmas 34.25.1 and 34.20.11,

  3. locally finite type, see Lemmas 34.25.2 and 34.20.10,

  4. universally open, see Lemmas 34.25.4 and 34.20.4,

  5. syntomic, see Lemmas 34.26.1 and 34.20.26,

  6. smooth, see Lemmas 34.27.1 and 34.20.27,

  7. étale, see Lemmas 34.28.1 and 34.20.29,

  8. locally quasi-finite, see Lemmas 34.28.2 and 34.20.24,

  9. unramified, see Lemmas 34.28.3 and 34.20.28,

  10. G-unramified, see Lemmas 34.28.3 and 34.20.28, and

  11. add more here as needed.

Remark 34.29.8. At this point we have three possible definitions of what it means for a property $\mathcal{P}$ of morphisms to be “étale local on the source and target”:

  1. $\mathcal{P}$ is étale local on the source and $\mathcal{P}$ is étale local on the target,

  2. (the definition in the paper [Page 100, DM] by Deligne and Mumford) for every diagram

    \[ \xymatrix{ U \ar[d]_ a \ar[r]_ h & V \ar[d]^ b \\ X \ar[r]^ f & Y } \]

    with surjective étale vertical arrows we have $\mathcal{P}(h) \Leftrightarrow \mathcal{P}(f)$, and

  3. $\mathcal{P}$ is étale local on the source-and-target.

In this section we have seen that (SP) $\Rightarrow $ (DM) $\Rightarrow $ (ST). The Examples 34.29.1 and 34.29.2 show that neither implication can be reversed. Finally, Lemma 34.29.6 shows that the difference disappears when looking at properties of morphisms which are stable under postcomposing with open immersions, which in practice will always be the case.

Lemma 34.29.9. Let $\mathcal{P}$ be a property of morphisms of schemes which is étale local on the source-and-target. Given a commutative diagram of schemes

\[ \vcenter { \xymatrix{ X' \ar[d]_{g'} \ar[r]_{f'} & Y' \ar[d]^ g \\ X \ar[r]^ f & Y } } \quad \text{with points}\quad \vcenter { \xymatrix{ x' \ar[d] \ar[r] & y' \ar[d] \\ x \ar[r] & y } } \]

such that $g'$ is étale at $x'$ and $g$ is étale at $y'$, then $x \in W(f) \Leftrightarrow x' \in W(f')$ where $W(-)$ is as in Lemma 34.23.3.

Proof. Lemma 34.23.3 applies since $\mathcal{P}$ is étale local on the source by Lemma 34.29.4.

Assume $x \in W(f)$. Let $U' \subset X'$ and $V' \subset Y'$ be open neighbourhoods of $x'$ and $y'$ such that $f'(U') \subset V'$, $g'(U') \subset W(f)$ and $g'|_{U'}$ and $g|_{V'}$ are étale. Then $f \circ g'|_{U'} = g \circ f'|_{U'}$ has $\mathcal{P}$ by property (1) of Definition 34.29.3. Then $f'|_{U'} : U' \to V'$ has property $\mathcal{P}$ by (4) of Lemma 34.29.4. Then by (3) of Lemma 34.29.4 we conclude that $f'_{U'} : U' \to Y'$ has $\mathcal{P}$. Hence $U' \subset W(f')$ by definition. Hence $x' \in W(f')$.

Assume $x' \in W(f')$. Let $U' \subset X'$ and $V' \subset Y'$ be open neighbourhoods of $x'$ and $y'$ such that $f'(U') \subset V'$, $U' \subset W(f')$ and $g'|_{U'}$ and $g|_{V'}$ are étale. Then $U' \to Y'$ has $\mathcal{P}$ by definition of $W(f')$. Then $U' \to V'$ has $\mathcal{P}$ by (4) of Lemma 34.29.4. Then $U' \to Y$ has $\mathcal{P}$ by (3) of Lemma 34.29.4. Let $U \subset X$ be the image of the étale (hence open) morphism $g'|_ U' : U' \to X$. Then $\{ U' \to U\} $ is an étale covering and we conclude that $U \to Y$ has $\mathcal{P}$ by (1) of Lemma 34.29.4. Thus $U \subset W(f)$ by definition. Hence $x \in W(f)$. $\square$

Lemma 34.29.10. Let $k$ be a field. Let $n \geq 2$. For $1 \leq i, j \leq n$ with $i \not= j$ and $d \geq 0$ denote $T_{i, j, d}$ the automorphism of $\mathbf{A}^ n_ k$ given in coordinates by

\[ (x_1, \ldots , x_ n) \longmapsto (x_1, \ldots , x_{i - 1}, x_ i + x_ j^ d, x_{i + 1}, \ldots , x_ n) \]

Let $W \subset \mathbf{A}^ n_ k$ be a nonempty open subscheme such that $T_{i, j, d}(W) = W$ for all $i, j, d$ as above. Then either $W = \mathbf{A}^ n_ k$ or the characteristic of $k$ is $p > 0$ and $\mathbf{A}^ n_ k \setminus W$ is a finite set of closed points whose coordinates are algebraic over $\mathbf{F}_ p$.

Proof. We may replace $k$ by any extension field in order to prove this. Let $Z$ be an irreducible component of $\mathbf{A}^ n_ k \setminus W$. Assume $\dim (Z) \geq 1$, to get a contradiction. Then there exists an extension field $k'/k$ and a $k'$-valued point $\xi = (\xi _1, \ldots , \xi _ n) \in (k')^ n$ of $Z_{k'} \subset \mathbf{A}^ n_{k'}$ such that at least one of $x_1, \ldots , x_ n$ is transcendental over the prime field. Claim: the orbit of $\xi $ under the group generated by the transformations $T_{i, j, d}$ is Zariski dense in $\mathbf{A}^ n_{k'}$. The claim will give the desired contradiction.

If the characteristic of $k'$ is zero, then already the operators $T_{i, j, 0}$ will be enough since these transform $\xi $ into the points

\[ (\xi _1 + a_1, \ldots , \xi _ n + a_ n) \]

for arbitrary $(a_1, \ldots , a_ n) \in \mathbf{Z}_{\geq 0}^ n$. If the characteristic is $p > 0$, we may assume after renumbering that $\xi _ n$ is transcendental over $\mathbf{F}_ p$. By successively applying the operators $T_{i, n, d}$ for $i < n$ we see the orbit of $\xi $ contains the elements

\[ (\xi _1 + P_1(\xi _ n), \ldots , \xi _{n - 1} + P_{n - 1}(\xi _ n), \xi _ n) \]

for arbitrary $(P_1, \ldots , P_{n - 1}) \in \mathbf{F}_ p[t]$. Thus the Zariski closure of the orbit contains the coordinate hyperplane $x_ n = \xi _ n$. Repeating the argument with a different coordinate, we conclude that the Zariski closure contains $x_ i = \xi _ i + P(\xi _ n)$ for any $P \in \mathbf{F}_ p[t]$ such that $\xi _ i + P(\xi _ n)$ is transcendental over $\mathbf{F}_ p$. Since there are infinitely many such $P$ the claim follows.

Of course the argument in the preceding paragraph also applies if $Z = \{ z\} $ has dimension $0$ and the coordinates of $z$ in $\kappa (z)$ are not algebraic over $\mathbf{F}_ p$. The lemma follows. $\square$

Lemma 34.29.11. Let $\mathcal{P}$ be a property of morphisms of schemes. Assume

  1. $\mathcal{P}$ is étale local on the source,

  2. $\mathcal{P}$ is smooth local on the target,

  3. $\mathcal{P}$ is stable under postcomposing with open immersions: if $f : X \to Y$ has $\mathcal{P}$ and $Y \subset Z$ is an open subscheme then $X \to Z$ has $\mathcal{P}$.

Given a commutative diagram of schemes

\[ \vcenter { \xymatrix{ X' \ar[d]_{g'} \ar[r]_{f'} & Y' \ar[d]^ g \\ X \ar[r]^ f & Y } } \quad \text{with points}\quad \vcenter { \xymatrix{ x' \ar[d] \ar[r] & y' \ar[d] \\ x \ar[r] & y } } \]

such that $g$ is smooth $y'$ and $X' \to X \times _ Y Y'$ is étale at $x'$, then $x \in W(f) \Leftrightarrow x' \in W(f')$ where $W(-)$ is as in Lemma 34.23.3.

Proof. Since $\mathcal{P}$ is étale local on the source we see that $x \in W(f)$ if and only if the image of $x$ in $X \times _ Y Y'$ is in $W(X \times _ Y Y' \to Y')$. Hence we may assume the diagram in the lemma is cartesian.

Assume $x \in W(f)$. Since $\mathcal{P}$ is smooth local on the target we see that $(g')^{-1}W(f) = W(f) \times _ Y Y' \to Y'$ has $\mathcal{P}$. Hence $(g')^{-1}W(f) \subset W(f')$. We conclude $x' \in W(f')$.

Assume $x' \in W(f')$. For any open neighbourhood $V' \subset Y'$ of $y'$ we may replace $Y'$ by $V'$ and $X'$ by $U' = (f')^{-1}V'$ because $V' \to Y'$ is smooth and hence the base change $W(f') \cap U' \to V'$ of $W(f') \to Y'$ has property $\mathcal{P}$. Thus we may assume there exists an étale morphism $Y' \to \mathbf{A}^ n_ Y$ over $Y$, see Morphisms, Lemma 28.34.20. Picture

\[ \xymatrix{ X' \ar[r] \ar[d] & Y' \ar[d] \\ \mathbf{A}^ n_ X \ar[r]_{f_ n} \ar[d] & \mathbf{A}^ n_ Y \ar[d] \\ X \ar[r]^ f & Y } \]

By Lemma 34.29.6 (and because étale coverings are smooth coverings) we see that $\mathcal{P}$ is étale local on the source-and-target. By Lemma 34.29.9 we see that $W(f')$ is the inverse image of the open $W(f_ n) \subset \mathbf{A}^ n_ X$. In particular $W(f_ n)$ contains a point lying over $x$. After replacing $X$ by the image of $W(f_ n)$ (which is open) we may assume $W(f_ n) \to X$ is surjective. Claim: $W(f_ n) = \mathbf{A}^ n_ X$. The claim implies $f$ has $\mathcal{P}$ as $\mathcal{P}$ is local in the smooth topology and $\{ \mathbf{A}^ n_ Y \to Y\} $ is a smooth covering.

Essentially, the claim follows as $W(f_ n) \subset \mathbf{A}^ n_ X$ is a “translation invariant” open which meets every fibre of $\mathbf{A}^ n_ X \to X$. However, to produce an argument along these lines one has to do étale localization on $Y$ to produce enough translations and it becomes a bit annoying. Instead we use the automorphisms of Lemma 34.29.10 and étale morphisms of affine spaces. We may assume $n \geq 2$. Namely, if $n = 0$, then we are done. If $n = 1$, then we consider the diagram

\[ \xymatrix{ \mathbf{A}^2_ X \ar[r]_{f_2} \ar[d]_ p & \mathbf{A}^2_ Y \ar[d] \\ \mathbf{A}^1_ X \ar[r]^{f_1} & \mathbf{A}^1_ Y } \]

We have $p^{-1}(W(f_1)) \subset W(f_2)$ (see first paragraph of the proof). Thus $W(f_2) \to X$ is still surjective and we may work with $f_2$. Assume $n \geq 2$.

For any $1 \leq i, j \leq n$ with $i \not= j$ and $d \geq 0$ denote $T_{i, j, d}$ the automorphism of $\mathbf{A}^ n$ defined in Lemma 34.29.10. Then we get a commutative diagram

\[ \xymatrix{ \mathbf{A}^ n_ X \ar[r]_{f_ n} \ar[d]_{T_{i, j, d}} & \mathbf{A}^ n_ Y \ar[d]^{T_{i, j, d}} \\ \mathbf{A}^ n_ X \ar[r]^{f_ n} & \mathbf{A}^ n_ Y } \]

whose vertical arrows are isomorphisms. We conclude that $T_{i, j, d}(W(f_ n)) = W(f_ n)$. Applying Lemma 34.29.10 we conclude for any $x \in X$ the fibre $W(f_ n)_ x \subset \mathbf{A}^ n_ x$ is either $\mathbf{A}^ n_ x$ (this is what we want) or $\kappa (x)$ has characteristic $p > 0$ and $W(f_ n)_ x$ is the complement of a finite set $Z_ x \subset \mathbf{A}^ n_ x$ of closed points. The second possibility cannot occur. Namely, consider the morphism $T_ p : \mathbf{A}^ n \to \mathbf{A}^ n$ given by

\[ (x_1, \ldots , x_ n) \mapsto (x_1 - x_1^ p, \ldots , x_ n - x_ n^ p) \]

As above we get a commutative diagram

\[ \xymatrix{ \mathbf{A}^ n_ X \ar[r]_{f_ n} \ar[d]_{T_ p} & \mathbf{A}^ n_ Y \ar[d]^{T_ p} \\ \mathbf{A}^ n_ X \ar[r]^{f_ n} & \mathbf{A}^ n_ Y } \]

The morphism $T_ p : \mathbf{A}^ n_ X \to \mathbf{A}^ n_ X$ is étale at every point lying over $x$ and the morphism $T_ p : \mathbf{A}^ n_ Y \to \mathbf{A}^ n_ Y$ is étale at every point lying over the image of $x$ in $Y$. (Details omitted; hint: compute the derivatives.) We conclude that

\[ T_ p^{-1}(W) \cap \mathbf{A}^ n_ x = W \cap \mathbf{A}^ n_ x \]

by Lemma 34.29.9 (we've already seen $\mathcal{P}$ is étale local on the source-and-target). Since $T_ p : \mathbf{A}^ n_ x \to \mathbf{A}^ n_ x$ is finite étale of degree $p^ n > 1$ we see that if $Z_ x$ is not empty then it contains $T_ p^{-1}(Z_ x)$ which is bigger. This contradiction finishes the proof. $\square$


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