A topos (plural topoi, toposes) is a category that behaves like the category of sheaves of sets on a topological space. Topos theory consists of the study of Grothendieck topoi, used in algebraic geometry, and the study of elementary topoi, used in logic.

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Is there a reasonable Grothendieck topology on the category of modules over a ring?

How about over a field (i.e. f.d vector spaces)? Can these categories be considered as a site?
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What does it take to divide by $2$?

Theorem 1 [ZFC, classical logic]: If $A,B$ are sets such that $\textbf{2}\times A\cong \textbf{2}\times B$, then $A\cong B$. That's because the axiom of choice allows for the definition of ...
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Could there be an “$n$-th root” of the category $\mathsf{Set}$?

Here is a thought experiment: Suppose we did not know what sets and functions are. The general idea of a topos is, that it somehow serves as a foundation for mathematics. So let there be an alternate ...
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Moving from sheaves over spaces to sheaves over sites

The first example of a sheaf that I have consciously come across is the sheaf of continuous (real) functions on some topological space. The fact it is a sheaf is equivalent to the pasting lemma, which ...
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295 views

Sheaves in Grothendieck Topologies

Let $S$ be a scheme and the category of $S$-schemes be equipped with one of the standard Grothendieck topologies, say étale or fppf. Let $G \rightarrow H$ be a morphism of abelian sheaves on this ...
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Examples of toposes in which the Axiom of Determinacy holds.

I just stumbled upon the Axiom of Determinacy which is an axiom in set theory - inconsistent with the Axiom of Choice, consistent with the Axiom of Dependent Choice, that states that for every subset ...
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56 views

Constructive proof that the Lie functor is faithful?

I am wondering how to show that the Lie functor taking Lie groups to Lie algebras is faithful? In particular, I am looking for a constructive proof, since I am working in the context of synthetic ...
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Notation for inhabited sets

A set $X$ is called inhabited if it has some element. In classical mathematics, this means that it is not the empty set, so that one usually writes $X \neq \emptyset$. However, in intuitionistic ...
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33 views

Internal semantics of a category based on a fixed topos.

I'm not certain as to how I should formulate this question; it might be considered a soft question. I am interested in finding a general way to take a category $\mathbb{C}$ and an (elementary) topos ...
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Is every topos equivalent to a full subtopos of U-small objects in another topos?

Let $\mathcal{E}$ be a topos, and $\mathcal{U}$ an $\mathcal{E}$-universe (as discussed on this nLab page). Let $\mathcal{E}_\mathcal{U}$ be the full subcategory of $\mathcal{U}$-small objects in ...
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Description of generated Grothendieck topology

Let $C$ be a small category, and let $\tau$ be a set of sieves in $C$. Assume that $\tau$ contains all the maximal sieves, and is stable under pullbacks. How to describe the Grothendieck topology ...
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How does the internal language of a topos come to be?

There are several books and articles on topos theory which mention the internal language, but I can't manage to see the big picture from any of them. I would like a soft explanation of how the ...
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Category-theoretic properties of cardinals

Let $\kappa$ be a cardinal, let $\mathbf{H}_\kappa$ be the set of hereditarily $\kappa$-small sets, and let $\mathbf{Set}_{< \kappa}$ be the full subcategory of $\mathbf{Set}$ corresponding to ...
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191 views

Can curvature be defined in Topos Theory?

I'm by no means an expert in either category theory or topos theory, but I'm trying to gain some perspective on traditional geometric ideas in this context. Topos theory claims that it is a geometric ...
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44 views

Covering sieves in a Grothendieck topology

I'm trying to get my head around some of the basics of Grothendieck topologies. Let $(\mathcal{C}, J)$ be a site, let $U$ be an object of $\mathcal{C}$ and let $J(U)$ be the set of covering sieves on ...
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What does it mean for a topos to be “generated” by some kind of objects?

Is there a universal definition of what that phrase should mean? Suppose we are considering objects of a topos with a particular property, call them $P$-objects (I have in mind the case where ...
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68 views

Stack semantics for the layman

I am trying to read bits and pieces of Ingo Blechschmidt's notes on using the internal language of toposes in algebraic geometry. I have not studied the internal language. I only have a bare bones ...
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Replacement in a topos with an eye to a natural model of TST

TST is a typed first order set theory that is essentially the friendly, simple version of Russell's type theory. There are an infinite number of sorts indexed by the natural numbers, each variable ...
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topos defined by sets

Let $C=Sets$ be the category/site of sets, equipped with the topology defined by surjective families. Why is the associated topos $T$ equivalent to the punctual topos $Sh(pt)\simeq Sets$? (This is ...
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I want to prove that $\text{Sh}(C_{\mathbb{T}},J)$ is the classifing topos for the theory of $\mathbb{T}$-local algebras.

Let $\mathbb{T}$ be a essentially algebraic theory, $C_{\mathbb{T}}$ be its syntactic category and $J$ be a subcanonical coverage on $C_{\mathbb{T}}$ Then, I want to understand why ...
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29 views

Enough injectives in a topos

In the category of sets, each nonempty set is injective since given a mono $A\ \rightarrowtail B$ and an arrow $A\rightarrow C$ we can lift to an arrow $B\rightarrow C$ by giving up injectivity : send ...
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Gerbes and Brauer group

Let $µ$ be a sheaf of abelian groups on a site $C$. There is a bijection between isomorphism (equivalence) classes of µ-gerbes over $C$ and $H^2 (C, µ)$. Can someone give me a good reference for the ...
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'Smooth' p-adic analysis (perhaps via toposes)

There are sensible theories of analytic functions on non-Archimedean fields (rigid analytic spaces, Berkovich spaces), but these are modeled after complex analysis. I'm curious to what extent there ...
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topos have colimits

Define an (elementary) topos to be a cartesian closed category with all finite limits and subobject classifiers. I'm looking for a proof of the fact that a topos also has all finite colimits. I know ...
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When does an atomic topos have this property?

If $f:\mathbb{E}\to\mathbf{Set}$ is an atomic (or locally connected if you prefer) Grothendieck topos, when is it the case that the direct image functor $f_*:\mathbb{E}\to \mathbf{Set}$ is faithful?
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Example of a small topos

I'm currently trying to understand this article by T. Noll on the topos of triads in music theory (also, this) However, I can't get past section 2.2 where Noll introduces the subobject classifier, ...
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Finite Limits, Exponentiation and Sub-Object Classifiers imply Finite Co-Limits

I recently started studying topoi and the book I am using defines them as categories that have all finite limits and co-limits, exponentiation and sub-object classifiers. The book briefly remarks that ...
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What are some topos-theoretic insights about $G$-sets?

Since a $G$-set is just a functor $G\longrightarrow \mathsf{Set}$, the category of $G$-sets seems to be a simple example of a topos. What are some topos-theoretic insights into $G$-sets? Insights ...
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The category of presheaves on a possibly-large category

Suppose $\mathcal{C}$ is a category such that for every $c \in \mathrm{Ob}(\mathcal{C})$, the slice category $\mathcal{C}/c$ is equivalent to a small category. I need to show that the category of ...
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Is the reflector $\mathcal{E} \rightarrow \text{Sep}_j \mathcal{E}$ left exact? (j is a local operator)

According to Mac Lane & Moerdijk, Ch V, Ex 4, the left adjoint $\mathcal{E} \rightarrow \text{Sep}_j\,\mathcal{E}$ of the inclusion is left exact, but the proof is left as an exercise for the ...
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W-types and inverse image functor [closed]

Since the question did not get an answer here I have posted it to mathoverflow at http://mathoverflow.net/questions/218855/w-types-and-inverse-image-functor All sheaf topoi have W-types and in fact ...
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Technical meaning of “profinite circle”

In a private exchange with a professional mathematician, I found the following statement: the "small etale topos" of a finite field is a "profinite circle", and thus looks like circle. Could anyone ...
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Is there another well-pointed elementary topos satisfying internal choice without natural numbers object?

Is there an elementary topos which Is well-pointed Satisfies the internal axiom of choice Does not have a natural numbers object; and Is not the category of finite sets?
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Applications of category theory and topoi/topos theory in reality

I am an amateur mathematician with an interest in the subjects named in the title. I have recently come to understand that my B.A. in math gives me absolutely no qualification at all in the Swedish ...
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subobject classifier and characteristic arrow in presheaf topos

I'm reading the page 54 of this pdf but don't understand the definition of characteristic arrow in $\text{Set}^{\mathbb{C}^{\text{op}}}$. Assume that $F, G \in \text{Set}^{\mathbb{C}^{\text{op}}}$, ...
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Terminal object in $Set^{C^{op}}$ and subobject classifier.

This is from Sheaves in Geometry and Logic pg 38. I'm not sure if I understood it correctly but the subobject classifier in $Set^{C^{op}}$ when $C$ is a small category is a map (natural ...
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What is the “internal language of a topos”?

What does the sentence "[...] these statements should be interpreted, of course, in the internal language of the topos $\mathcal{E}$" mean, in the context of, say, the definition of a groupoid in ...
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What is Mazzola's “Topos of Music” about?

Disclaimers: I am neither a musician, nor I want to discredit Mazzola's work. Corollary of the first point: please use a plain style, without technical terms in the area of Music Theory. Corollary of ...
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When does Sheafification commute with direct image?

Given a presheaf $\mathcal{F}$ on a space $X$ and a map $f: X \rightarrow Y$, when does $f_* A(\mathcal{F}) = A(f_* \mathcal{F})$, where $A$ is the associated sheaf/sheafification functor? Since ...
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About the relation between the categories $\text{Sch}$, $\text{LRS}$ and $\text{RS}$

I've been trying to find some useful categorical facts about the category of schemes, locally ringed spaces and ringed spaces (that I shall denote by $\text{Sch}$, $\text{LRS}$ and $\text{RS}$ ...
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A question on a property of geometric morphisms related to locales.

Is the "localic reflection" of a geometric morphism between topoi the same thing as its "localic part"?
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Is this assignment of the topos of sheaves functorial?

Let $\mathcal{C}$ be a site and for any object $X$ of $\mathcal{C}$ denote by $\text{Sh}(X)$ the category of sheaves on the site $\mathcal{C}/X$. My question is, what can we say about this assignment? ...
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Can I define a site as a category endowed with a pretopology instead of a topology?

If $K$ is a pretopology on a category $\mathcal{C}$ and $J$ the topology it induces, are the Grothendieck toposes $\text{Sh}(\mathcal{C},K)$ and $\text{Sh}(\mathcal{C},J)$ the same in general? As I ...
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What are the universally effective epimorphisms of topological spaces?

An effective epimorphism in a category is a morphism that is the coequaliser of its kernel pair, and a universally effective epimorphism is a morphism $f : X \to Y$ such that, for every pullback ...
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The Relationship between Separable Functors and Faithful Functors

Consider the adjunction $\mathcal{C} \mathrel{\substack{\mathcal{F}\\\rightleftarrows\\ \mathcal{G}}} \mathcal{D} $ together with unit $\eta: I_{\!_{\mathcal{C}}} \rightarrow \mathcal{G} \mathcal{F} $ ...
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Does the inverse image sheaf have a left adjoint for $\mathsf{Set}$-valued sheaves?

It's known that for sheaves with values in modules, the inverse image sheaf functor $j^\ast$ for $j:U\subset X$ an inclusion of an open set has a left adjoint which is extension by zero. Is there any ...
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Does an equivalence of $G$-sets and $H$-sets imply an isomorphism of $G$ and $H$?

Here $G$-sets denote the category of sets which have a left $G$-action. So the question is whether a functor $F \colon \text{$G$-sets} \to \text{$H$-sets}$ implies that we have an isomorphism of ...
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Simple question in “Sheaves in geometry and logic”

There's an argument I don't understand in "Sheaves in geometry and logic" by Mac Lane and Moerdijk, that seems a priori easy but I can't see it. Page 174, diagram (10) (involving the power ...
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base change of an equivalence relation of fppf sheaves

Let $S$ be a scheme, $R,U$ be $S$-schemes and $s,t : R \to U \times_S U$ be an equivalence relation i.e. it's a monomorphisme such that for every $S$-scheme $T$, $R(T) \to U(T) \times U(T)$ is and ...
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Fibrations over topoi

Let $\mathcal{S}$ be an elementary topos. What is (exactly) the relation between $\mathcal{S}$-indexed categories and fibrations over $\mathcal{S}$? Where can I read about this? (Or even find the ...