Section 100.7 (04YE): Properties of algebraic stacks defined by properties of schemes

100.7 Properties of algebraic stacks defined by properties of schemes

Any smooth local property of schemes gives rise to a corresponding property of algebraic stacks via the following lemma. Note that a property of schemes which is smooth local is also étale local as any étale covering is also a smooth covering. Hence for a smooth local property $P$ of schemes we know what it means to say that an algebraic space has $P$ , see Properties of Spaces, Section 66.7.

Lemma 100.7.1. Let $\mathcal{P}$ be a property of schemes which is local in the smooth topology, see Descent, Definition 35.15.1. Let $\mathcal{X}$ be an algebraic stack. The following are equivalent

for some scheme $U$ and some surjective smooth morphism $U \to \mathcal{X}$ the scheme $U$ has property $\mathcal{P}$ ,
for every scheme $U$ and every smooth morphism $U \to \mathcal{X}$ the scheme $U$ has property $\mathcal{P}$ ,
for some algebraic space $U$ and some surjective smooth morphism $U \to \mathcal{X}$ the algebraic space $U$ has property $\mathcal{P}$ , and
for every algebraic space $U$ and every smooth morphism $U \to \mathcal{X}$ the algebraic space $U$ has property $\mathcal{P}$ .

If $\mathcal{X}$ is a scheme this is equivalent to $\mathcal{P}(U)$ . If $\mathcal{X}$ is an algebraic space this is equivalent to $X$ having property $\mathcal{P}$ .

Proof. Let $U \to \mathcal{X}$ surjective and smooth with $U$ an algebraic space. Let $V \to \mathcal{X}$ be a smooth morphism with $V$ an algebraic space. Choose schemes $U'$ and $V'$ and surjective étale morphisms $U' \to U$ and $V' \to V$ . Finally, choose a scheme $W$ and a surjective étale morphism $W \to V' \times _\mathcal {X} U'$ . Then $W \to V'$ and $W \to U'$ are smooth morphisms of schemes as compositions of étale and smooth morphisms of algebraic spaces, see Morphisms of Spaces, Lemmas 67.39.6 and 67.37.2. Moreover, $W \to V'$ is surjective as $U' \to \mathcal{X}$ is surjective. Hence, we have

$\mathcal{P}(U) \Leftrightarrow \mathcal{P}(U') \Rightarrow \mathcal{P}(W) \Rightarrow \mathcal{P}(V') \Leftrightarrow \mathcal{P}(V)$

where the equivalences are by definition of property $\mathcal{P}$ for algebraic spaces, and the two implications come from Descent, Definition 35.15.1. This proves (3) $\Rightarrow$ (4).

The implications (2) $\Rightarrow$ (1), (1) $\Rightarrow$ (3), and (4) $\Rightarrow$ (2) are immediate. $\square$

Definition 100.7.2. Let $\mathcal{X}$ be an algebraic stack. Let $\mathcal{P}$ be a property of schemes which is local in the smooth topology. We say $\mathcal{X}$ has property $\mathcal{P}$ if any of the equivalent conditions of Lemma 100.7.1 hold.

Remark 100.7.3. Here is a list of properties which are local for the smooth topology (keep in mind that the fpqc, fppf, and syntomic topologies are stronger than the smooth topology):

locally Noetherian, see Descent, Lemma 35.16.1,
Jacobson, see Descent, Lemma 35.16.2,
locally Noetherian and $(S_ k)$ , see Descent, Lemma 35.17.1,
Cohen-Macaulay, see Descent, Lemma 35.17.2,
reduced, see Descent, Lemma 35.18.1,
normal, see Descent, Lemma 35.18.2,
locally Noetherian and $(R_ k)$ , see Descent, Lemma 35.18.3,
regular, see Descent, Lemma 35.18.4,
Nagata, see Descent, Lemma 35.18.5.

Any smooth local property of germs of schemes gives rise to a corresponding property of algebraic stacks. Note that a property of germs which is smooth local is also étale local. Hence for a smooth local property of germs of schemes $P$ we know what it means to say that an algebraic space $X$ has property $P$ at $x \in |X|$ , see Properties of Spaces, Section 100.7.

Lemma 100.7.4. Let $\mathcal{X}$ be an algebraic stack. Let $x \in |\mathcal{X}|$ be a point of $\mathcal{X}$ . Let $\mathcal{P}$ be a property of germs of schemes which is smooth local, see Descent, Definition 35.21.1. The following are equivalent

for any smooth morphism $U \to \mathcal{X}$ with $U$ a scheme and $u \in U$ with $a(u) = x$ we have $\mathcal{P}(U, u)$ ,
for some smooth morphism $U \to \mathcal{X}$ with $U$ a scheme and some $u \in U$ with $a(u) = x$ we have $\mathcal{P}(U, u)$ ,
for any smooth morphism $U \to \mathcal{X}$ with $U$ an algebraic space and $u \in |U|$ with $a(u) = x$ the algebraic space $U$ has property $\mathcal{P}$ at $u$ , and
for some smooth morphism $U \to \mathcal{X}$ with $U$ a an algebraic space and some $u \in |U|$ with $a(u) = x$ the algebraic space $U$ has property $\mathcal{P}$ at $u$ .

If $\mathcal{X}$ is representable, then this is equivalent to $\mathcal{P}(\mathcal{X}, x)$ . If $\mathcal{X}$ is an algebraic space then this is equivalent to $\mathcal{X}$ having property $\mathcal{P}$ at $x$ .

Proof. Let $a : U \to \mathcal{X}$ and $u \in |U|$ as in (3). Let $b : V \to \mathcal{X}$ be another smooth morphism with $V$ an algebraic space and $v \in |V|$ with $b(v) = x$ also. Choose a scheme $U'$ , an étale morphism $U' \to U$ and $u' \in U'$ mapping to $u$ . Choose a scheme $V'$ , an étale morphism $V' \to V$ and $v' \in V'$ mapping to $v$ . By Lemma 100.4.3 there exists a point $\overline{w} \in |V' \times _\mathcal {X} U'|$ mapping to $u'$ and $v'$ . Choose a scheme $W$ and a surjective étale morphism $W \to V' \times _\mathcal {X} U'$ . We may choose a $w \in |W|$ mapping to $\overline{w}$ (see Properties of Spaces, Lemma 66.4.4). Then $W \to V'$ and $W \to U'$ are smooth morphisms of schemes as compositions of étale and smooth morphisms of algebraic spaces, see Morphisms of Spaces, Lemmas 67.39.6 and 67.37.2. Hence

$\mathcal{P}(U, u) \Leftrightarrow \mathcal{P}(U', u') \Leftrightarrow \mathcal{P}(W, w) \Leftrightarrow \mathcal{P}(V', v') \Leftrightarrow \mathcal{P}(V, v)$

The outer two equivalences by Properties of Spaces, Definition 66.7.5 and the other two by what it means to be a smooth local property of germs of schemes. This proves (4) $\Rightarrow$ (3).

The implications (1) $\Rightarrow$ (2), (2) $\Rightarrow$ (4), and (3) $\Rightarrow$ (1) are immediate. $\square$

Definition 100.7.5. Let $\mathcal{P}$ be a property of germs of schemes which is smooth local. Let $\mathcal{X}$ be an algebraic stack. Let $x \in |\mathcal{X}|$ . We say $\mathcal{X}$ has property $\mathcal{P}$ at $x$ if any of the equivalent conditions of Lemma 100.7.4 holds.

Comments (2)

Comment #2678 by denis lieberman on July 31, 2017 at 18:17

Properties of spaces reference to 90.7 at the end of the first paragraph should be to 57.7

Comment #2834 by Johan on October 04, 2017 at 13:07

Thanks, fixed here.

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The Stacks project

100.7 Properties of algebraic stacks defined by properties of schemes

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