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1. • CommentRowNumber1.
   • CommentAuthorUrs
   • CommentTimeMay 26th 2020
   • PermaLink
   Author: Urs
   Format: MarkdownItexI have removed the few stub lines that were in this entry before, and wrote out a more informative Idea-section. Currently it reads as follows: *** In [[general topology|basic topology]] and [[differential geometry]], by an _atlas_ of/for a [[topological manifold|topological]]-, [[differentiable manifold|differentiable]]- or [[smooth manifold]] $X$ one means a collection of [[coordinate charts]] $U_i \subset X$ which form an [[open cover]] of $X$. If one considers here the [[disjoint union]] $\mathcal{U} \coloneqq \underset{i}{\sqcup} U_i$ of all the choordinate charts, then the separate chart embeddings $U_i \subset X$ give rise to a single [[map]] ([[continuous function|continuous]]/[[differentiable function]]) $$ \mathcal{U} \longrightarrow X $$ and now the condition for an atlas is that this is a [[surjective function|surjective]] [[étale map]]/[[local diffeomorphism]]. If, next, one regards this morphism, under the [[Yoneda embedding]], inside the [[topos]] of [[formal smooth sets]], then these conditions on an atlas say that this morphism is 1. an [[effective epimorphism]]; 1. a [[formally étale morphism]]. In this abstract form the concept of an atlas generalizes to any [[cohesion|cohesive]] [[higher geometry]] ([KS 17, Def. 3.3](#KhavkineSchreiber17), [Wellen 18, Def 4.13](#Wellen18)). Next, for a [[geometric stack]] $\mathcal{X}$, an atlas is a [[smooth manifold]] $\mathcal{U}$ (for [[differentiable stacks]]) or [[scheme]] $\mathcal{U}$ (for [[algebraic stacks]]) or similar, equipped with a morphism $$ \mathcal{U} \longrightarrow \mathcal{X} $$ that is an [[effective epimorphism]] and [[formally étale morphism]] in the corresponding [[(infinity,1)-topos|higher topos]] (for instance in that of [[formal smooth infinity-groupoids]]). Here the terminology has a bifurcation: 1. In the general context of [[geometric stacks]] one typically drops the second condition and calls any [[effective epimorphism]] from a [[smooth manifold]] or [[scheme]] to a [[differentiable stack]] or [[algebraic stack]], respectively, an _atlas_. 1. If in addition the condition is imposed that such an effective epimorphism exists which is also [[formally étale morphism|formally étale]], then the [[geometric stack]] is called an _[[orbifold]]_ or _[[Deligne-Mumford stack]]_ (often with various further conditions imposed). *** <a href="https://ncatlab.org/nlab/revision/diff/atlas/5">diff</a>, <a href="https://ncatlab.org/nlab/revision/atlas/5">v5</a>, <a href="https://ncatlab.org/nlab/show/atlas">current</a>

   I have removed the few stub lines that were in this entry before, and wrote out a more informative Idea-section. Currently it reads as follows:

   ---

   In basic topology and differential geometry, by an atlas of/for a topological-, differentiable- or smooth manifold $X$ one means a collection of coordinate charts $U_i \subset X$ which form an open cover of $X$.

   If one considers here the disjoint union $\mathcal{U} \coloneqq \underset{i}{\sqcup} U_i$ of all the choordinate charts, then the separate chart embeddings $U_i \subset X$ give rise to a single map (continuous/differentiable function)

   $$\mathcal{U} \longrightarrow X$$

   and now the condition for an atlas is that this is a surjective étale map/local diffeomorphism.

   If, next, one regards this morphism, under the Yoneda embedding, inside the topos of formal smooth sets, then these conditions on an atlas say that this morphism is

   1. an effective epimorphism;

   2. a formally étale morphism.

   In this abstract form the concept of an atlas generalizes to any cohesive higher geometry (KS 17, Def. 3.3, Wellen 18, Def 4.13).

   Next, for a geometric stack $\mathcal{X}$, an atlas is a smooth manifold $\mathcal{U}$ (for differentiable stacks) or scheme $\mathcal{U}$ (for algebraic stacks) or similar, equipped with a morphism

   $$\mathcal{U} \longrightarrow \mathcal{X}$$

   that is an effective epimorphism and formally étale morphism in the corresponding higher topos (for instance in that of formal smooth infinity-groupoids).

   Here the terminology has a bifurcation:

   1. In the general context of geometric stacks one typically drops the second condition and calls any effective epimorphism from a smooth manifold or scheme to a differentiable stack or algebraic stack, respectively, an atlas.

   2. If in addition the condition is imposed that such an effective epimorphism exists which is also formally étale, then the geometric stack is called an orbifold or Deligne-Mumford stack (often with various further conditions imposed).

   ---

   diff, v5, current

2. • CommentRowNumber2.
   • CommentAuthorUrs
   • CommentTimeNov 2nd 2021
   • PermaLink
   Author: Urs
   Format: MarkdownItexcross-linked with the new entry *[[groupoid objects in an (∞,1)-topos are effective]]* <a href="https://ncatlab.org/nlab/revision/diff/atlas/9">diff</a>, <a href="https://ncatlab.org/nlab/revision/atlas/9">v9</a>, <a href="https://ncatlab.org/nlab/show/atlas">current</a>

   cross-linked with the new entry groupoid objects in an (∞,1)-topos are effective

   diff, v9, current

3. • CommentRowNumber3.
   • CommentAuthorUrs
   • CommentTimeSep 18th 2022
   • (edited Sep 18th 2022)
   • PermaLink
   Author: Urs
   Format: MarkdownItexadded a paragraph ([here](https://ncatlab.org/nlab/show/atlas#AtlasForACategory)) cross-linking with *[[category with an atlas]]* and *[[flagged category]]*. <a href="https://ncatlab.org/nlab/revision/diff/atlas/11">diff</a>, <a href="https://ncatlab.org/nlab/revision/atlas/11">v11</a>, <a href="https://ncatlab.org/nlab/show/atlas">current</a>

   added a paragraph (here) cross-linking with category with an atlas and flagged category.

   diff, v11, current