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3
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1answer
173 views

Projectives and Injectives in Functor Categories

Would it be possible to enlighten me (or even better give a reference) about enough projectives (injectives) in functor categories? Here is a precise question. Let $C$ be a small category, whose ...
5
votes
1answer
304 views

Can one characterize the category of finite-dimensional vector spaces? [duplicate]

Let $K$ be a field. Does the category of finitely generated $K$-modules have a nice characterization, for example as the unique abelian category satisfying a certain simple condition? For example, we ...
2
votes
0answers
56 views

Quotient categories and essential extension

Let $A$ be a right Noetherian positively graded ring. Let $Gr(A)$ be the category of right graded $A$-modules, and $Tors(A)$ be the full subcategory of $Gr(A)$ of torsion modules. Let $QGr(A)$ be the ...
6
votes
1answer
230 views

About an embedding of abelian categories into categories of modules

Let $k$ be a field. Let $C$ be an abelian $k$-linear category with a symmetric tensor product $\otimes$ and internal homomorphisms, such that $\mathrm{End}(1)=k$. Let $M$ be another $k$-linear abelian ...
6
votes
0answers
203 views

Extension to a right exact functor

Setup. Let $k$ be a field, $\mathcal{C}$ be a $k$-linear abelian category, $\mathcal{D}$ an arbitrary $k$-linear category. Let $\mathcal{C}'$ be a full subcategory of $\mathcal{C}$ with the following ...
8
votes
0answers
334 views

Deligne-Mumford Stacks and exactness of products

Let me start by saying that this is probably a very ingenuous question but I still need to digest most definitions before being able to formulate a more precise question. In this paper: ...
3
votes
0answers
160 views

Cisinski-Deglise model structure on chain complexes

Let $\newcommand{\A}{\mathscr{A}}\A$ be a grothendieck abelian category. In their paper "Local and stable homological algebra in grothendieck abelian categories", Cisinski and Déglise define the ...
2
votes
2answers
349 views

how to make the category of chain complexes into an $(\infty,1)$-category

Related to this question, I would like to know, if there is an explicit presentation of the $(\infty,1)$-category of a model category of abelian chain complexes, but this time in terms of simplicial ...
6
votes
2answers
437 views

Properties of quotient categories.

I asked this on math.stackexchange.com, but didn't get any answer. Let $\mathcal{A}$ be an abelian category and $\mathcal{C}$ a localizing subcategory in the sense of Gabriel. (A Serre/thick/dense ...
1
vote
0answers
105 views

Why are the left exact functors from an abelian category to abelian groups cocomplete and have a injective generator?

Let $\mathcal{C}$ be an abelian category, $\mathcal{Ab}$ the category of abelian groups and $Lex(\mathcal{C}, \mathcal{B})$ the category of left exact functors between abelian categories. What is the ...
3
votes
2answers
286 views

Applications and examples of quotient categories of abelian categories

I want to get motivated to learn more about quotient categories of abelian categories by a Serre subcategory or even by a localizing category as they are described in Pierre Gabriel's thesis "Des ...
4
votes
2answers
284 views

Why is $Lex(\mathcal{A},\mathcal{Ab})$ abelian? Does $Lex(\mathcal{A},\mathcal{Ab})\rightarrow Func(\mathcal{A},\mathcal{Ab})$ admit a left-adjoint?

What is the best way to show, that $Lex(\mathcal{A},\mathcal{Ab})$ is abelian, where $\mathcal{A}$ is an abelian category and $\mathcal{Ab}$ is the category of abelian groups from scratch? There is ...
9
votes
1answer
396 views

Examples of applications of the Freyd-Mitchell embedding theorem.

The Freyd-Mitchell embedding theorem states the following: Let $\mathcal{A}$ be a small abelian category. There exists a unital ring $R$ and a full, faithful and exact functor ...
7
votes
2answers
338 views

Recovering an abelian category out of its derived category

I'm trying to learn more about derived category stuff and my curiosity has made me to ask these questions. Sorry if I'm being sloppy, I'm a new learner. In Wikipedia it has been stated that since ...
2
votes
1answer
361 views

Any abelian category as filtered colimit of categories of projective modules

Recently I have heard somewhere that any (edit: small) abelian category can be expressed as the colimit of categories of projective modules over some rings. The remark was that this is "basically just ...
1
vote
1answer
207 views

Why does tensor product in Ab(V) require colimits in V?

In Tom Leinster's book on operads, he gives Ab(V), the category of abelian groups in a symmetric monoidal category V, as an example of a multicategory that doesn't arise from a monoidal category, ...
27
votes
5answers
2k views

Cocomplete but not complete abelian category

This is a duplicate of the following question to which I did not receive any answer: http://math.stackexchange.com/questions/238247/complete-but-not-cocomplete-category Let $\mathfrak C$ be an ...
1
vote
0answers
252 views

Additive functors preserving quasi-isomorphism

Let $F: \mathcal{A} \rightarrow \mathcal{B}$ be an additive functor between abelian categories (with enough injectives and projectives) and $K^\cdot, L^\cdot$ objects of $\textrm{Ch}(\mathcal{A})$. ...
3
votes
1answer
297 views

A finite diagram in an abelian category which may not be locally small

This question was posted in StackExchange, but there has been no answer so far. This question is motivated by this. I will use the notations of my answer to this. We say a category $\mathcal C$ is ...
2
votes
0answers
132 views

Does mapping cylinder category have enough injectives?

Let $A, B$ be two abelian categories, and $\tau : A \to B$ a left exact functor. We define a category $C$ as follows: objects: triples $(M, N, \varphi)$ where $M\in A, N\in B$ and $\varphi: N\to ...
11
votes
5answers
1k views

Cov. right-exact additive functors that don't commute with direct sums?

Background Recently, I have been writing up some notes on derived functors and I came across the Eilenberg-Watts theorems [1], which essentially explain why it is hard to find derived functors ...
2
votes
2answers
504 views

Subcategories of abelian categories generated by finitely many objects

Hello! I am trying to understand the structure of the smallest abelian subcategory of an abelian category that contains one object $X$ and all endomorphisms of that object (or rather containing a ...
10
votes
1answer
547 views

Higher “Cartan-Eilenberg” Resolutions

I am a number theory graduate student learning a bit of homological algebra, and I am curious about higher complexes in abelian categories. I apologize if my post is slightly vague as I am not an ...
5
votes
1answer
461 views

Is the bounded derived category of coherent sheaves of a variety a small category?

The question is in the title. I am trying to apply the Mitchell (Freyd-Mitchell?) embedding theorem, which states that for every small abelian category $A$, there exists a ring $R$ such that A ...
5
votes
1answer
342 views

Morita equivalence of acyclic categories

(Crossposted from math.SE.) Call a category acyclic if only the identity morphisms are invertible and the endomorphism monoid of every object is trivial. Let $C, D$ be two finite acyclic categories. ...
0
votes
1answer
604 views

Terminology - subcategories of Abelian categories

Hello, I have terminological question. Consider the following properties of a full subcategory $B \subset A$, where $A$ is an abelian category, and we assume $B$ to be closed under finite direct ...
7
votes
3answers
595 views

Module category equivalent to graded module category?

Main Question Let $R$ be a graded ring, graded by the nonnegative integers. Denote by $\mathrm{gr}R-\mathrm{Mod}$ the category of $\mathbb{Z}$-graded left $R$-modules with morphisms that preserve ...
7
votes
2answers
718 views

The composition of derived functors - commutation fails hazardly?

Hello, When we have left exact functors $F: A \to B , G: B \to C$ (between abelian categories), we would like sometimes to state that $D(GF)=D(G)D(F)$ (functors between bounded below derived ...
3
votes
2answers
546 views

Indecomposable projectives correspond to irreducibles - reference

Hello, We have the following assertion: In an abelian category that has enough projectives and in which every object has finite length, indecomposable projectives correspond bijectively to ...
7
votes
2answers
468 views

Incarnations of a theorem of Eilenberg

Let $R$ be any ring, let $\text{Mod}_R$ be the category of right $R$-modules and let $\text{Ab}$ be the category of abelian groups. There is a classical theorem of Eilenberg (I think) which says that ...
2
votes
1answer
128 views

Categories with canonical factorizations into products satisfying two particular properties

An old splitting theorem for (Hausdorff) locally compact abelian (LCA) groups says that any LCA group $L$ is isomorphic to a direct product of $\mathbb{R}^n$ and $L_1$, where $L_1$ contains a ...
7
votes
0answers
251 views

DG vs. abelian quotients

The following, if true, should probably be "standard," but I don't know where to look. I'd rather be slightly imprecise about hypotheses in the hope that there's a good general answer. Feel free to ...
3
votes
1answer
297 views

Is the colimit of a filtered diagram of module categories in AbCat induced by a cofiltered diagram of rings equivalent to the category of modules over the limit?

Given a cofiltered diagram of commutative rings $F:D\to \mathrm{CRing}$, we obtain a filtered diagram $\mathrm{Mod}(F):D^{op}\to \mathrm{ExAbCat}$ (where $\mathrm{ExAbCat}$ is the category of Abelian ...
3
votes
1answer
479 views

Tame abelian tensor categories

In the article "Tannaka duality for geometric stacks" (arxiv, see nlab for a summary) Jacob Lurie introduced the notion of a tame abelian tensor category. An abelian tensor category is called tame if ...
12
votes
3answers
1k views

Is there an additive functor between abelian categories which isn't exact in the middle?

Suppose $F: C\to D$ is an additive functor between abelian categories and that $$0\to X\xrightarrow f Y\xrightarrow g Z\to 0$$ is and exact sequence in $C$. Does it follow that ...
4
votes
4answers
932 views

The category of abelian group objects

Let $C$ be a category, say with finite products. What can be said about the category $Ab(C)$ of abelian group objects of $C$? Is it always an abelian category? If not, what assumptions on $C$ have to ...
6
votes
2answers
425 views

What are examples of cogenerators in R-mod?

Fill in the blank, please :) Let $\mathcal C$ be a complete and cocomplete abelian category. A generator in $\mathcal C$ is an object $X \in \mathcal C$ such that every object $Y \in \mathcal C$ ...
9
votes
1answer
766 views

Mitchell's embedding theorem

Mitchell's embedding theorem http://en.wikipedia.org/wiki/Mitchell%27s_embedding_theorem tells us that every small abelian category ${\cal A}$ has a full, faithful and exact embedding $V : {\cal A} ...
1
vote
1answer
1k views

F(0) = 0? F: additive functor

If I define an additive functor to be a functor on abelian categories such that the action of F on Hom(A,B) is a group homomorphism, do I necessarily have that F(zero object) = zero object?
1
vote
2answers
389 views

Tot and colimits

This must be a well-known exercise with spectral sequences, but I don't know a reference for it. I'm trying to figure out when does $Tot$ commute with colimits. More precisely, let $X$ be a double ...
7
votes
3answers
554 views

Moduli of Extensions

Given two modules $M$ and $N$ there is a nice scheme parametrizing extensions $0 \rightarrow M \rightarrow E \rightarrow N \rightarrow 0$ namely $Ext^1(N,M)$ or, leaving out the trivial extension, ...
4
votes
2answers
481 views

Does the Grothendieck group depend on the embedding?

This might turn out to be a silly question, but here goes. Let $\mathcal{C}$ be a full additive subcategory of an abelian category $\mathcal{A}$. I'm wondering if the Grothendieck group ...
9
votes
2answers
382 views

Failure of Fin. Presented and Fin. Generated Modules to be Abelian Categories?

Let R be a ring. I'm trying to understand when the categories of finitely presented R-modules and finitely generated R-modules can fail to be abelian categories. Poking around on the internet has ...
4
votes
1answer
799 views

Is the chain homotopy category, K(Ab), an Abelian category? By Ab, I mean the category of Abelian groups.

Let A be an Abelian category. From this category, we can form the chain complex category Ch(A). The objects of Ch(A) are chain complexes of objects of A. The morphisms of Ch(A) are chain maps. ...
11
votes
3answers
804 views

How cavalier can I be when demanding a category have direct sums?

In my meaning, a direct sum in a category should really be called a "biproduct". If $X,Y$ are objects, then a direct sum $X \oplus Y$ is an object $Z$ along with isomorphisms $\hom(Z,A) = \hom(X,A) ...
1
vote
1answer
235 views

Does the product (by an object) in an abelian category ever have a right adjoint?

This is a follow-up to this question. Since an abelian category cannot be cartesian closed, clearly the hom functor is not right adjoint to the product (by an object). However, does the product (by ...
11
votes
1answer
1k views

Can a topos ever be an abelian category?

Can a topos ever be a nontrivial abelian category? If not, where does the contradiction lie? If a topos can be an abelian category, can you give a (notrivial!) example?
4
votes
3answers
874 views

Existence of projective resolutions in abelian categories

It is a standard result of elementary homological algebra that to every R-module $A$ there exists a projective resolution. It is often said that the category of R-modules has "enough projectives." ...
18
votes
2answers
720 views

Is there an infinity × infinity lemma for abelian categories?

Many people know that there is a (3×3) nine lemma in category theory. There is also apparently a sixteen lemma, as used in a paper on the arXiv (see page 24). There might be a twenty-five lemma, as ...