Processing math: 100%

The Stacks project

Lemma 101.23.1. Let f : \mathcal{X} \to \mathcal{Y} be a morphism of algebraic stacks. Assume f is representable by algebraic spaces. The following are equivalent

  1. f is locally quasi-finite (as in Properties of Stacks, Section 100.3), and

  2. f is locally of finite type and for every morphism \mathop{\mathrm{Spec}}(k) \to \mathcal{Y} where k is a field the space |\mathop{\mathrm{Spec}}(k) \times _\mathcal {Y} \mathcal{X}| is discrete.

Proof. Assume (1). In this case the morphism of algebraic spaces \mathcal{X}_ k \to \mathop{\mathrm{Spec}}(k) is locally quasi-finite as a base change of f. Hence |\mathcal{X}_ k| is discrete by Morphisms of Spaces, Lemma 67.27.5. Conversely, assume (2). Pick a surjective smooth morphism V \to \mathcal{Y} where V is a scheme. It suffices to show that the morphism of algebraic spaces V \times _\mathcal {Y} \mathcal{X} \to V is locally quasi-finite, see Properties of Stacks, Lemma 100.3.3. The morphism V \times _\mathcal {Y} \mathcal{X} \to V is locally of finite type by assumption. For any morphism \mathop{\mathrm{Spec}}(k) \to V where k is a field

\mathop{\mathrm{Spec}}(k) \times _ V (V \times _\mathcal {Y} \mathcal{X}) = \mathop{\mathrm{Spec}}(k) \times _\mathcal {Y} \mathcal{X}

has a discrete space of points by assumption. Hence we conclude that V \times _\mathcal {Y} \mathcal{X} \to V is locally quasi-finite by Morphisms of Spaces, Lemma 67.27.5. \square


Comments (0)

There are also:

  • 5 comment(s) on Section 101.23: Locally quasi-finite morphisms

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.