# Questions tagged [topos-theory]

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### Why do elementary topoi have pullbacks?

In the book of Szabo "Algebra of Proofs", Definition 13.1.9 introduces an elementary topos as a cartesian closed category with a subobject classifier. On the other hand, many other sources including ...
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### Examples of Heyting categories that are not toposes?

When explaining how Heyting categories can model first order logic it would be nice to be able to give some small example and contrast it with Set-semantics. I realized however that I don't know of ...
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### Topos extensions

In set theory, starting from a model $V$ of $ZFC$, a forcing notion $\mathbb{P}$, and a generic filter $G \subset \mathbb{P}$ over $V$, we can find a generic extension which is a model of $ZFC$ and is ...
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### Results of geometric model theory

I read in some places that model theory has useful applications in algebraic geometry. Could someone give me some results that come from applying model theory to algebraic geometry? I also saw that ...
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### Classifying toposes of theories of rings that aren't local rings

The standard uses of toposes in algebraic geometry come from sites that look roughly like the syntactic sites of theories of local rings that they classify. This isn't particularly surprising, since ...
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### Toposes in which countable choice is true but dependent choice isn't

I'd like examples of toposes in which Countable Choice is true but Dependent Choice isn't. I'd prefer examples without Excluded Middle. It's hard to find a natural example.
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### Characterization of geometric morphisms without referring explicitly to the left adjoint?

Recall that a functor $f_\ast : \mathcal E \to \mathcal F$ between toposes is called a geometric morphism if it has a left exact left adjoint $f^\ast$. Is there an intrinsic characterization of such ...
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I have several questions about geometric morphisms of topoi. It was recommended that I move my question here from Math.Stackexchange, since it would be good to get an expert on topos theory to answer. ...
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### When is an $\infty$-categorical localization left exact?

Let $L: \mathcal C^\to_\leftarrow L\mathcal C : i$ be an adjunction with $i$ fully faithful. In ordinary category theory, $L$ is left exact iff the class of $L$-local morphisms is stable under base ...
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### Only discrete topology gives trivial topos?

Given a Grothendieck site $\mathsf{(C,\tau)}$, if the associated Grothendieck topos $\mathsf{Shv(C,\tau)}$ is trivial, i.e. $\mathsf{Shv(C,\tau)}$ consists of the terminal sheaf $*$ only, can we ...
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### A list of locally finitely presentable topoi that are not coherent

Coherent topoi play an important role in topos theory, especially in the interaction with logic. Their most handy characterization is provided by Johnstone. Sketches, D3.3.1. Every coherent topos is ...
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### An axiom that shows that the real numbers are weakly countable?

Is there a model of Intuitionistic Higher-Order Logic in which the following axiom is true? Covering Axiom: Any true statement of the form $\forall x \in A, \exists y \in B, \phi(x,y)$ gives rise to ...
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### What's an example of an $\infty$-topos not equivalent to sheaves on a Grothendieck site?

My question is as in the title: Does anyone have an example (supposing one exists) of an $\infty$-topos which is known not to be equivalent to sheaves on a Grothendieck site. An $\infty$-topos is as ...
239 views

### Non-linear Galois descent

This question is about Galois theory. So let $K / k$ be a Galois extension of fields. Let us assume that $K / k$ is finite dimensional, though everything can be made to work in the profinite case by ...
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### Is there an internal notion of “flat continuous presheaf”?

I have asked a version of this question here, but have been unable to receive an answer or devise one of my own, and am venturing that it may be appropriate for Math Overflow. The "internal form" of ...
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### 'Continuity' of the étale topos

In certain concrete situations, I can show that the small étale topos of an inverse limit of schemes is the inverse limit of the associated toposes, for example, if $X$ is a (qcqs) scheme relative ...
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### Has this “backwards” perspective on toposes been studied?

Topos theory can be seen as a categorification of topology via the following analogies. \begin{array}{|c|c|} \hline \text{locales}&\text{Grothendieck toposes}\\\hline \text{open sets}&\text{...
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### Geometric morphism whose counit is epic

Is there a name for the class of geometric morphisms whose counits are epic (equivalently, whose direct images are faithful)? This notably includes the class of inclusions/embeddings of toposes. By ...
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### Is well-pointedness the reason that the internal/external distinction seems not to apply to $\mathbf{Set}$?

When reasoning about the category of sets, we usually don't have to worry about the internal/external distinction. For example, if $f : X \rightarrow Y$ is a morphism of sets, then $f$ is either ...
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### What are the uses of topoi in algebraic geometry today?

Since Grothendieck introduced them in the 1960s, topoi have found many applications in algebraic geometry, category theory, and logic. For instance, they appear in the development of étale and ...
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### Are constant $\infty$-sheaves constant on connected components?

Let $C$ be an $\infty$-category endowed with a Grothendieck topology $J$ and consider the $\infty$-topos $\infty\text{Sh}(C, J)$. There is a natural geometric morphism to $\infty\text{Grpd}$ whose ...
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### Failure of SVC in Grothendieck toposes

The axiom SVC (for "small violations of choice") asserts that there is a set $S$ such that for every set $X$ there is a choice set $A$ such that $X$ is a subquotient of (i.e. admits a surjection from ...
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### A categorial PCF theory?

I'm not an expert in PCF theory, so please forgive me if this question makes no sense. I'm looking for a categorial version of PCF theory. Specifically, if we replace $Set$ with another category, ...
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### Examples of topos that are not ordinary spaces

In [SGA6] we find: Mais nous lui conseillons néanmoins, de préférence, de s'assimiler le langage des topos, qui fournit un principe d'unification extrêmement commode. (DeepL translate: However, we ...
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### A list of proofs of “Coherent topoi have enough points”

For my research I would like to read all the known proofs of the very classical result "Coherent topoi have enough points", by Deligne. Ref 1: D3.3.13 in Sketches of an Elephant provides a very ...
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### What is the logical progression in algebraic tools for studying spaces (varieties -> schemes, sheaves, topos etc.)?

Some algebraists (Cartier, Weil, Atiyah, etc.) sometimes speak of geometry as a long history of essentially asking the same question—"what is space, and how would one describe a space uniquely". ...
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### Fibrant objects in Bousfield localization of homotopy pullback closure of Nisnevich hypercovers

Let $M$ be a model topos and $S$ a set of morphisms, there exists a set of morphism $\bar{S}$ which is generated by the $S$-local equivalences which is closed under homotopy pullbacks in $M$. Suppose ...
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### O. Leroy's thesis on fundamental groupoids

Does someone have a copy of the O. Leroy's thesis: Groupoïde fondamental et théorème de van Kampen en théorie des topos or has the ability to make a digitalization ? The thesis was done at ...
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### Does the 1-category construction of a topos of presheaves extend to the 2-Category of Groupoids?

In the case of 1-categories, we know there is a functor category $PSh(C):=[C^{op},Set]$, where $C$ is a small category, and this functor category is a topos. I am hoping this will extend to the case ...
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### Can the effective topos be seen as symmetric monoidal?

In Example(s) of monoidal symmetric closed category with NNO without infinite coproducts? user Zhen Lin states the effective topos is locally cartesian closed. On nLab we have that locally ...
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### Topos over an Accessible and Cocomplete category

Some facts have been drawn to my attention that raise the following two conjectures to my mind: Many good properties of domains, dcpos, are captured categorically by accessible, cocomplete categories....
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### Reference request about “internal language of categories”

I've tried to become familiar with the so-called "internal language of a category" for the last months. However, I'm still not confident enough when, for instance, I find a subobject (of a given ...
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### Is every Grothendieck category with a generator a category of sheaves?

The Gabriel-Popescu theorem tells us that every Grothendieck category with a generator is a left exact localization of a module category. I'm interested in a slightly different way of "representing" ...
Let ${\cal A}$ be the category of finitely presented $\mathbb{C}$-algebras. Let $J$ be the largest subcanonical Grothendieck topology on ${{\cal A}^{op}}$ such that the local algebras in $\cal A$ are ...