## 93.13 Algebraic stacks and algebraic spaces

In this section we discuss some simple criteria which imply that an algebraic stack is an algebraic space. The main result is that this happens exactly when objects of fibre categories have no nontrivial automorphisms. This is not a triviality! Before we come to this we first do a sanity check.

Lemma 93.13.1. Let $S$ be a scheme contained in $\mathit{Sch}_{fppf}$.

1. A category fibred in groupoids $p : \mathcal{X} \to (\mathit{Sch}/S)_{fppf}$ which is representable by an algebraic space is a Deligne-Mumford stack.

2. If $F$ is an algebraic space over $S$, then the associated category fibred in groupoids $p : \mathcal{S}_ F \to (\mathit{Sch}/S)_{fppf}$ is a Deligne-Mumford stack.

3. If $X \in \mathop{\mathrm{Ob}}\nolimits ((\mathit{Sch}/S)_{fppf})$, then $(\mathit{Sch}/X)_{fppf} \to (\mathit{Sch}/S)_{fppf}$ is a Deligne-Mumford stack.

Proof. It is clear that (2) implies (3). Parts (1) and (2) are equivalent by Lemma 93.12.4. Hence it suffices to prove (2). First, we note that $\mathcal{S}_ F$ is stack in sets since $F$ is a sheaf (Stacks, Lemma 8.6.3). A fortiori it is a stack in groupoids. Second the diagonal morphism $\mathcal{S}_ F \to \mathcal{S}_ F \times \mathcal{S}_ F$ is the same as the morphism $\mathcal{S}_ F \to \mathcal{S}_{F \times F}$ which comes from the diagonal of $F$. Hence this is representable by algebraic spaces according to Lemma 93.9.4. Actually it is even representable (by schemes), as the diagonal of an algebraic space is representable, but we do not need this. Let $U$ be a scheme and let $h_ U \to F$ be a surjective étale morphism. We may think of this as a surjective étale morphism of algebraic spaces. Hence by Lemma 93.10.3 the corresponding $1$-morphism $(\mathit{Sch}/U)_{fppf} \to \mathcal{S}_ F$ is surjective and étale. $\square$

The following result says that a Deligne-Mumford stack whose inertia is trivial “is” an algebraic space. This lemma will be obsoleted by the stronger Proposition 93.13.3 below which says that this holds more generally for algebraic stacks...

Lemma 93.13.2. Let $S$ be a scheme contained in $\mathit{Sch}_{fppf}$. Let $\mathcal{X}$ be an algebraic stack over $S$. The following are equivalent

1. $\mathcal{X}$ is a Deligne-Mumford stack and is a stack in setoids,

2. $\mathcal{X}$ is a Deligne-Mumford stack such that the canonical $1$-morphism $\mathcal{I}_\mathcal {X} \to \mathcal{X}$ is an equivalence, and

3. $\mathcal{X}$ is representable by an algebraic space.

Proof. The equivalence of (1) and (2) follows from Stacks, Lemma 8.7.2. The implication (3) $\Rightarrow$ (1) follows from Lemma 93.13.1. Finally, assume (1). By Stacks, Lemma 8.6.3 there exists a sheaf $F$ on $(\mathit{Sch}/S)_{fppf}$ and an equivalence $j : \mathcal{X} \to \mathcal{S}_ F$. By Lemma 93.9.5 the fact that $\Delta _\mathcal {X}$ is representable by algebraic spaces, means that $\Delta _ F : F \to F \times F$ is representable by algebraic spaces. Let $U$ be a scheme, and let $x : (\mathit{Sch}/U)_{fppf} \to \mathcal{X}$ be a surjective étale morphism. The composition $j \circ x : (\mathit{Sch}/U)_{fppf} \to \mathcal{S}_ F$ corresponds to a morphism $h_ U \to F$ of sheaves. By Bootstrap, Lemma 79.5.1 this morphism is representable by algebraic spaces. Hence by Lemma 93.10.4 we conclude that $h_ U \to F$ is surjective and étale. Finally, we apply Bootstrap, Theorem 79.6.1 to see that $F$ is an algebraic space. $\square$

Proposition 93.13.3. Let $S$ be a scheme contained in $\mathit{Sch}_{fppf}$. Let $\mathcal{X}$ be an algebraic stack over $S$. The following are equivalent

1. $\mathcal{X}$ is a stack in setoids,

2. the canonical $1$-morphism $\mathcal{I}_\mathcal {X} \to \mathcal{X}$ is an equivalence, and

3. $\mathcal{X}$ is representable by an algebraic space.

Proof. The equivalence of (1) and (2) follows from Stacks, Lemma 8.7.2. The implication (3) $\Rightarrow$ (1) follows from Lemma 93.13.2. Finally, assume (1). By Stacks, Lemma 8.6.3 there exists an equivalence $j : \mathcal{X} \to \mathcal{S}_ F$ where $F$ is a sheaf on $(\mathit{Sch}/S)_{fppf}$. By Lemma 93.9.5 the fact that $\Delta _\mathcal {X}$ is representable by algebraic spaces, means that $\Delta _ F : F \to F \times F$ is representable by algebraic spaces. Let $U$ be a scheme and let $x : (\mathit{Sch}/U)_{fppf} \to \mathcal{X}$ be a surjective smooth morphism. The composition $j \circ x : (\mathit{Sch}/U)_{fppf} \to \mathcal{S}_ F$ corresponds to a morphism $h_ U \to F$ of sheaves. By Bootstrap, Lemma 79.5.1 this morphism is representable by algebraic spaces. Hence by Lemma 93.10.4 we conclude that $h_ U \to F$ is surjective and smooth. In particular it is surjective, flat and locally of finite presentation (by Lemma 93.10.9 and the fact that a smooth morphism of algebraic spaces is flat and locally of finite presentation, see Morphisms of Spaces, Lemmas 66.37.5 and 66.37.7). Finally, we apply Bootstrap, Theorem 79.10.1 to see that $F$ is an algebraic space. $\square$

Comment #4875 by on

Minor typo: in the proof of Lemma 03YS, it should say 'We may think of this as a surjective étale morphism of algebraic spaces'.

Comment #7775 by Mingchen on

I suggest to include https://math.stanford.edu/~conrad/papers/kmpaper.pdf 2.2.5 as well. This result is closely related to the topic of this section.

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