# Tagged Questions

Various structures are studied in category theory using properties of objects and morphisms between them. Many constructions are special cases of categorical limits and colimits (e.g. products in various categories). The notions of functor and natural transformations are very important in category ...

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### Good books and lecture notes about category theory.

What are the best books and lecture notes on category theory?
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### Category-theoretic limit related to topological limit?

Is there any connection between category-theoretic term 'limit' (=universal cone) over diagram, and topological term 'limit point' of a sequence, function, net...? To be more precise, is there a ...
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### Equivalent conditions for a preabelian category to be abelian

Let's fix some terminology first. A category $\mathcal{C}$ is preabelian if: 1) $Hom_{\mathcal{C}}(A,B)$ is an abelian group for every $A,B$ such that composition is biadditive, 2) $\mathcal{C}$ has ...
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### When to learn category theory?

I'm a undergraduate who wishes to learn category theory but I only have basic knowledge of linear algebra and set theory, I've also had a short course on number theory which used some basic concepts ...
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### In (relatively) simple words: What is an inverse limit?

I am a set theorist in my orientation, and while I did take a few courses that brushed upon categorical and algebraic constructions, one has always eluded me. The inverse limit. I tried to ask one of ...
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### Yoneda-Lemma as generalization of Cayleys theorem?

I came across the statement, that Yoneda-lemma is a generalization of Cayleys theorem which states, that every group is isomorphic to a group of permutations. How exactly does generalizes Yoneda-...
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### In categorical terms, why is there no canonical isomorphism from a finite dimensional vector space to its dual?

I've read in several places that one motivation for category theory was to be able to give precise meaning to statements like, "finite dimensional vector spaces are canonically isomorphic to their ...
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### What is a natural isomorphism?

I came across the adjective "natural" many times in my reading and I think this has something to do with category theory. Could someone please illustrate the idea behind this adjective to me? Many ...
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### What's the deal with empty models in first-order logic?

Asaf's answer here reminded me of something that should have been bothering me ever since I learned about it, but which I had more or less forgotten about. In first-order logic, there is a convention ...
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I'm trying to wrap my brain around adjoint functors. One of the examples I've seen is the categories $\bf IntLE \bf = (\mathbb{Z}, ≤)$ and $\bf RealLE \bf = (\mathbb{R}, ≤)$, where the ceiling functor ...
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### What is a universal property?

Sorry, but I do not understand the formal definition of "universal property" as given at Wikipedia. To make the following summary more readable I do equate "universal" with "initial" and omit the ...
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### Why are topological spaces interesting to study?

In introductory real analysis, I dealt only with $\mathbb{R}^n$. Then I saw that limits can be defined in more abstract spaces than $\mathbb{R}^n$, namely the metric spaces. This abstraction seemed "...
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### ''Labelling discrimination'' for objects in a category

I am not particularly used to the category theory thinking paradigm and there are certain statements that I am used to making in the set theoretic modelling approach that I would like to characterize ...
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### Analogue of the Cantor-Bernstein-Schroeder theorem for general algebraic structures

The Cantor-Bernstein-Schroeder theorem states that if there are two sets $A$ and $B$ such that there exist injective (alternatively, surjective, assuming choice I think) maps $A \to B$ and $B \to A$, ...
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### Categorial definition of subsets

I cannot see why this definition http://en.wikipedia.org/wiki/Subobject is equivalent to subsets in the category of sets. I am confused by the following facts, 1) the partial order is defined ...
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Recently, someone mentioned to me that given a function $f: X \to Y$ there are two natural functions between the powersets $P(X)$ and $P(Y)$. Namely $f: U \subset X \mapsto f(U)$ and $f^{-1}: V \... 1answer 1k views ### Unique up to unique isomorphism If an object$X$has a non-trivial automorphism$g$, for any isomorphism$f$with an object$Y$there is another isomorphism$f \circ g$between$X$and$Y$, so there is not a unique isomorphism ... 2answers 441 views ### Tensor product of monoids and arbitrary algebraic structures Question. Do you know a specific example which demonstrates that the tensor product of monoids (as defined below) is not associative? Let$C$be the category of algebraic structures of a fixed type,... 1answer 718 views ### Is an equivalence an adjunction? Let$C$and$D$be categories and$F:C\to D$,$G:D\to C$two functors.$F$is left-adjoint to$G$, if there are natural transformations$\eta:id_C\to GF$and$\epsilon:FG\to id_D$such that \begin{... 1answer 211 views ### Is it necessary for the homsets to be disjoint in a category? In defining a (locally small) category, some books include the condition that if$A\neq C$or$B\neq D$, then$\hom(A,B)$and$\hom(C,D)$are disjoint sets. Is it necessary? 2answers 121 views ### Categorical Pasting Lemma If I'm not mistaken, the pasting lemma for a two-element open cover$X=A\cup B$of a topological space is equivalent to saying that the following square is a pushout in$\mathsf{Top}$: $$\begin{matrix}... 1answer 350 views ### Injective Cogenerators in the Category of Modules over a Noetherian Ring Let R be a Noetherian ring and let \mathcal{A} be an injective R-module. The injectivity of \mathcal{A} is equivalent to the exactness of the functor Hom_R(-,\mathcal{A}), i.e. whenever we ... 1answer 311 views ### Do pullbacks commute with filtered limits in this sense? Let A_n, B_n, C_n be directed systems in some abelian category. Denote by A \times_C B the fibre product of A and B over C. Is it true that (\varprojlim A_n) \times_{\varprojlim C_n} (\... 4answers 1k views ### A bestiary about adjunctions What is your favourite adjoint? Following Mac Lane philosophy adjoints are everywhere, so I would like to draw a (possibly but unprobably) exhaustive list of adjunctions one faces in studying ... 1answer 3k views ### Quotient objects, their universal property and the isomorphism theorems This is a question that has been bothering me for quite a while. Let me put between quotation marks the terms that are used informally. "Quotient objects" are always the same. Take groups, abelian ... 1answer 426 views ### Rigidity of the category of fields Let's call a category rigid if every self-equivalence is isomorphic to the identity. For example, \mathsf{Set}, \mathsf{Grp}, \mathsf{Ab}, \mathsf{CRing} (MO/106838), \mathsf{Top} (SE/450193)... 1answer 1k views ### Functions f:\mathbb{N}\rightarrow \mathbb{Z} such that \left(m-n\right) \mid \left(f(m)-f(n)\right) A long time back, I wondered what functions other than integer polynomials on \mathbb{N} (or \mathbb{Z}) satisfied the property:$$\forall m,n: \left(m-n\right) \mid \left(f(m)-f(n)\right)$$... 3answers 726 views ### Mathematical structures Preamble: My previous education was focused either on classical analysis (which was given in quite old traditions, I guess) or on applied Mathematics. Since I was feeling lack of knowledge in 'modern' ... 2answers 1k views ### what are the product and coproduct in the category of topological groups I know the limits in the categories of groups, abelian groups and topological spaces and was wondering about the same thing. 1answer 699 views ### On the large cardinals foundations of categories (This was cross-posted to MathOverflow.) It is well-known that there are difficulties in developing basic category theory within the confines of$\sf ZFC$. One can overcome these problems when ... 1answer 4k views ### Was Grothendieck familiar with Stone's work on Boolean algebras? In short, my question is: Was Grothendieck familiar with Stone's work on Boolean algebras? Background: In an answer to Pierre-Yves Gaillard's question Did Zariski really define the Zariski ... 1answer 683 views ### Does$G\oplus G \cong H\oplus H$imply$G\cong H$in general? In this question, The Chaz asks whether$G\times G\cong H\times H$implies that$G\cong H$, where$G$and$H$are finite abelian groups. The answer is to his question is yes, by the structure theorem ... 1answer 875 views ### Adjoint functors as “conceptual inverses” The Stanford Encyclopedia of Philosophy's article on category theory claims that adjoint functors can be thought of as "conceptual inverses" of each other. For example, the forgetful functor "ought ... 4answers 1k views ### “The Yoneda embedding reflects exactness” is a direct consequence of Yoneda? Let$A,B,C$be objects of a category of modules over a ring. It is not hard to see that the Yoneda embedding "reflects exactness" (as Weibel puts it, on p. 28), i.e. if$\hom(X,A)\stackrel{f_*}{\to}\...
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In Awodey's book I read a slick proof that right adjoints preserve limits. If $F:\mathcal{C}\to \mathcal{D}$ and $G:\mathcal{D}\to \mathcal{C}$ is a pair of functors such that $(F,G)$ is an adjunction,...
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### Infinite coproduct of rings

I just learned from Wikipedia that coproduct of two (commutative) rings is given by tensor product over integers, and that coproduct of a family of rings is given by a "construction analogous to the ...
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I've recently started reading Categories for the Working Mathematician and I'm a little puzzled about the distinction between $\textbf{Set}$ and $\textbf{Ens}$. On the one hand, it seems like $\textbf{... 3answers 3k views ### Epimorphism and Monomorphism = Isomorphism? It seems to be that if a map is both an epimorphism and a monomorphism, it is not necessarily the case that it is an isomorphism. However, in the category of sets, if a map is both an epimorphism and ... 3answers 1k views ### Learning to think categorically (localization of rings and modules) I've been reading Ravi Vakil's notes on algebraic geometry notes and am having a hard time connecting the standard definition of the localization of a module to the definition in terms of universal ... 2answers 473 views ### What good are free groups? In Algebra: Chapter 0, one learns two definitions of Free Groups associating with sets. Let$A$be a set, the free group of$A$,$F(A)$is the initial object in the category$\mathcal{C}$, where \... 3answers 3k views ### Elements in$\hat{\mathbb{Z}}$, the profinite completion of the integers Let$\hat{\mathbb{Z}}$be the profinite completion of$\mathbb{Z}$. Since$\hat{\mathbb{Z}}$is the inverse limit of the rings$\mathbb{Z}/n\mathbb{Z}$, it's a subgroup of$\prod_n \mathbb{Z}/n\mathbb{...
Let $C$ and $D$ be categories and let $F : C \rightarrow D$, $G : C \rightarrow D$ be two functors such that they are either both covariant or both contravariant. Under what most general hypotheses is ...
Let $R$ be a ring with two-sided ideals $I,J$. The proof of the "absolute" chinese remainder theorem revolves around the fact that if $I,J$ cover $R$ in the lattice of ideals, i.e $I+J=R$, then the ...