Homological algebra studies homology in a general algebraic setting. The purpose is extraction of information about structures involved in terms of tangible objects like rings groups and modules.

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3
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29 views

Two short exact sequences with projective objects in the middle

Problem: Prove that for two short exact sequences $$ 0\rightarrow A \xrightarrow{f} B \xrightarrow{g} C \rightarrow 0 $$ $$ 0\rightarrow A' \xrightarrow{f'} B' \xrightarrow{g'} C \rightarrow 0, $$ ...
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7 views

How do I write the BRST-BV differential if I start with a dg Lie algebra and module?

The usual BRST-BV implements a Lie algebra and its module in terms of ghosts, etc. Where is there written a corresponding formula incorporating the differential of a dg Lie algebra and module? answer ...
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56 views

Question about Poincare duality and homology of a cylinder.

I am reading the paper. I have some questions about Poincare duality and homology of a cylinder. On page 9, example 2.6. Let $X = \mathbb{R} \times S^1$ be a cylinder and $Y = X/(0 \times S^1 )$, ...
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1answer
28 views

Projective resolution of ideal $\langle a,b\rangle $ in the $K=R[a,b]$

I saw a resolution as $$K\to K^2 \to \langle a,b\rangle \to 0,$$ but I can't figure out why, and can't figure out the maps. Could you give me some ideas? Thank you.
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0answers
16 views

Simplicial Complexes of Graphs notation question

I'm studying Jakob Jonsson's book Simplicial Complexes of Graphs very rigorously and in depth. I've been okay so far with the intensity and notation, but on page Chapter 3, section 2, page 30, I'm ...
4
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1answer
94 views

Values of the Herbrand quotient

For a finite cyclic group $G$, there is the Herbrand quotient in the theory of group cohomology. I calculated some of those quotients and I always came up with an Integer as solution. I failed at ...
0
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1answer
122 views

commutes homology

I am trying to prove the following: Let $R,A$ be rings and $\mathrm T:$$\mathscr M_R$ $\to $$\mathscr A_R$ such that $\mathscr M_R$ is category of left R modules and $\mathscr A_R$ is category of ...
3
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1answer
126 views

Classification of finitely generated multigraded modules over $K[x_1,\ldots,x_n]$?

Let $K$ be a field and $R=K[x_1,\ldots,x_n]=\bigoplus_{a\in\mathbb{N}^n}Kx^a$ the multigraded polynomial ring. Have finitely-generated multigraded $R$-modules been classified? Are they of the ...
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1answer
34 views

About a chain homotopy

Assume that $C=\lbrace C_{q},d_{q}\rbrace$ is a chain complex with each $C_{q}$ a free $R$-module. Let $C^{'}$ be another chain complex. Furthermore, assume that each $H_{q}$ is also free and that we ...
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0answers
45 views

Spectral sequences and Ext between extension of modules

Suppose $A$ is a commutative ring, $M_1,M_2,N_1,N_2$ are $A$-modules and we have two exact sequences of $A$-modules $$0\to M_1\to M\to M_2\to 0,$$ $$0\to N_1\to N\to N_2\to 0.$$ I want to write a ...
2
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2answers
52 views

diagram of short exact sequence

I have this commutative diagram of vector complex spaces where all the sequences that appear are short and exact. is there a way to say that $H$ is the intersection between $W1$ and $W2$?
3
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36 views

Tensor product of flat modules - proof verification

Let $A$ be a commutative ring, and let $B,C$ be commutative $A$-algebras. Let $M$ be a flat $B$-module and $N$ a flat $C$-module. I want to show that $M\otimes_A N$ is a flat $B\otimes_A C$-module. ...
0
votes
1answer
31 views

$p_k \colon M_k \to N_k$ is onto for $k>0$, if $p_0$ induces an iso on homology level, prove that $p_0$ is onto

We are working in $\textbf{Ch}_R$, chain complexes of $R-$modules. As the title suggest, I'm given a map (of chain complexes) $p\colon M \to N$ which is onto for $k>0$. It is known that ...
3
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1answer
50 views

Prove that $Ker(g \otimes k)= Im(f \otimes 1_{N}) + Im (1_{M} \otimes h)$

Suppose we have two short exact sequences: $$0 \to M' \mathrel{\overset{f}{\to}} M \mathrel{\overset{g}{\to}} M'' \to 0 $$ in Mod-R $$0 \to N' \mathrel{\overset{h}{\to}} N \mathrel{\overset{k}{\to}} ...
1
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1answer
49 views

Why is this Hilbert's Syzygy theorem?

In Lang's Algebra, chapter XXI, §4, on p. 861 he describes the standard construction of a graded (in principle infinite) free resolution of a finite graded module $M$ over the polynomial ring $A = ...
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0answers
33 views

If $Ext_A^n(M,N) \ne 0$, then $Ext_A^n(M,N') \ne 0$, for every indecomposable summand N' of N?

Let $A$ be an artin algebra and $M$ and $N$ finitely generated modules over $A$. Suppose that $Ext_A^n(M,N) \ne 0$, is it possible to conclude that $Ext_A^n(M,N') \ne 0$, for each indecomposable ...
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0answers
26 views

Reference for derived functor

I'm following a course in algebraic geometry and in 2-3 month we will see the cohomology of schemes using derived functors. I don't know anything about it, (and about category theory in general), ...
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0answers
37 views

Counterexample to exactness of functor from group representations to fixed points

I recently asked this question. Now, the answer there claimed that the functor $()^G:Rep_G\to Vect_{\mathbb{C}}$, where $Rep_G$ are complex representations of a group $G$, and $V^G=\{v\in V: ...
7
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1answer
60 views

Intuitive explanation of Four Lemma

In the Short Five Lemma where the rows are exact, it is a fact that $$\alpha \text{ and }\gamma \text{ injective (surjective) }\implies \beta \text{ injective (surjective)}.$$ I've heard this fact ...
2
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1answer
52 views

(An arbitrary direct product of free modules need not be free)

For each positive integer $i$ let $M_i$ be the free $\Bbb Z$-module $\Bbb Z$, and let $M$ be the direct product $\prod _{i \in \Bbb Z^+} M_i$. Each element of $M$ can be written uniquely in the form ...
3
votes
1answer
86 views

If the cohomology of two objects in the derived category are equal, are the objects isomorphic?

Let $\mathcal{A}$ be an abelian category. Given objects $A^\bullet,B^\bullet$ in the derived category $D(\mathcal{A})$. Assume that $H^n(A^\bullet)=H^n(B^\bullet)$ for all $n\in\mathbb{Z}$. Can we ...
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0answers
38 views

Shapiro's Lemma-Finding the inverse of an isomorphism.

Consider the isomorphism $\phi: H^n(G, Hom_{ZH}(ZG, A))\cong H^n(H,A)$ of shapiro's lemma. I would like to describe this via cochains. So the obvious map is $\phi(f+B^n(G,Hom_{ZH}(ZG, A) ...
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1answer
27 views

$M' \to M \to M'' \to 0$ exact $\implies 0\to \text{Hom}(M'',N) \to \text{Hom}(M,N) \to \text{Hom}(M',N)$ is exact.

Let, $M', M'', M, N$ be $A$-modules. $M' \stackrel{u}{\to} M \stackrel{v}{\to} M'' \to 0$ exact $\implies 0\to \text{Hom}(M'',N) \stackrel{\bar{v}}{\to} \text{Hom}(M,N) \stackrel{\bar{u}}{\to} ...
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1answer
38 views

Why is the torus not a boundary of a 3-chain?

I'm learning about homology right now and the author simply states that the torus $T^2$ does not have a boundary (I understand this) and also is not a boundary of a 3-chain. This is not at all obvious ...
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1answer
39 views

Extensions of quasicoherent sheaves are quasicoherent.

Harts theorem 5.7: Given an exact sequence $0 \to \mathscr F_1 \to \mathscr F_2 \to \mathscr F_3 \to 0 $ of sheaves on $X = \mathrm{spec} A$, if $\mathscr F_1$ and $\mathscr F_3$ are quasicoherent, ...
5
votes
3answers
127 views

Properties characterized by a vanishing Ext or Tor module

While reading Weibel's "An introduction to homological algebra'', I've noticed that many properties of a module are characterized by the vanishing of some Tor or Ext. Fix a (commutative) ring $R$ and ...
1
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1answer
34 views

what is the inclusion map for $Y$ to $Y$ x $Y$?

I am studying homotopy and homology and one map we have been using is the left and right inclusion maps $i_L$, $i_R$, for example from the space $Y$ to the cartesian product $Y$ x $Y$. Whilst I ...
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1answer
35 views

Modules in Morita Equivalence

In Method of Homological Algebra by Gelfand and Manin (Exercise 2.2.3). How are $\mathrm{Hom}_A(P,X)$ and $\mathrm{Hom}_B(P^*,Y)\,$ regarded as a $B$-module and $A$-module respectively?
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0answers
43 views

Characterization of Projective Objects

In which categories is an object $P$ projective if and only if every short exact sequence ending with it splits? $$0\longrightarrow A\longrightarrow B\longrightarrow P \longrightarrow 0$$
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1answer
28 views

Direct limit and constant are adjoint functors

I have a question. Why $(\varinjlim, | |)$ is an adjoint pair of functors? Here the definition of constant direct system || is: For any I, fix a module A and set $A_i=A$, all $i\in I$, and ...
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1answer
42 views

Defining a Map Between Two Chain Complexes

I would like someone to check my reasoning here and, if my reasoning is correct, help me define a map to make a short exact sequence. I am given a short exact sequence of chain complexes $$ ...
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0answers
21 views

Closure properties for classes of modules that form a cotorsion pair

A torsion theory is a pair of classes of $R$-modules (where $R$ is an associative ring with identity) $({\mathbb T},{\mathbb F})$, such that $r({\mathbb T})={\mathbb F}$ and $l({\mathbb F})={\mathbb ...
2
votes
3answers
102 views

Does a long exact sequence of flat modules remain exact after tensoring with an arbitrary module?

In Liu's Algebraic Geometry and Arithmetic Curves, Proposition 1.2.6 states that given any short exact sequence $0 \rightarrow M' \rightarrow M \rightarrow M'' \rightarrow 0$ with $M''$ flat, taking ...
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1answer
44 views

Projective dimension of module over local ring

This question arose reading the well known article by Buchsbaum Lectures on regular local rings. He states without proof that, given $(R,m)$ a local ring and an $R$-module $M$ over $R$, we have the ...
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0answers
38 views

The kernel of an antiderivation on an exterior algebra

This is a simple algebraic question I feel I should be obvious, but maybe isn't. Let $d'\colon V \twoheadrightarrow W$ be a surjective linear map of finite-dimensional vector spaces over a field of ...
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0answers
31 views

Derived projection formula

In the MO question here two "versions" of the projection formula are stated. The projection formula in algebraic geometry is, given a (quasicompact, quasiseparated) map of schemes $f: X \rightarrow ...
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1answer
60 views

Topological modules and relative homological algebra.

This question might be a bit dumb but I'm tired right now and this is just going over my head at the moment, in "The homology of Banach and topological algebras" Helemskii said that relative ...
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0answers
24 views

Bicomplexes-reference request

I'm not an expert in homological algebra, I would say that I have gathered only preliminary knowledge. I would like to learn more in particular about bicomplexes and homology and cohomology of such ...
0
votes
1answer
41 views

The hyper-derived functors $\mathbb L_\bullet F$ are just derived functors of $H_0F$?

Problem (Weibel's Introduction to Homological Algebra, Exercise 5.7.4,2) Let $\mathbf{Ch}_{\ge0}(\mathcal A)$ be the subcategory of complexes $A$ with $A_p=0$ for $p<0$. Then the hyper-derived ...
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0answers
65 views

A quasi-isomorphism between the total complex of a Cartan-Eilenberg resolution and the complex per se.

Problem (Weibel's Introduction to Homological Algebra, Exercise 5.7.1) Suppose $A$ is a (not necessarily bounded below) chain complex over an abelian category $\mathcal A$ where axiom (AB4) holds, ...
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28 views

How to compute cyclic/dihedral powers of modules?

The $n$-th cyclic power of an $R$-module $M$ is $$T^n_C(M)=M^{\otimes n}/\langle m_1\!\otimes\!m_2\!\otimes\!\ldots\!\otimes\!m_n-m_2\!\otimes\!\ldots\!\otimes\!m_n\!\otimes\!m_1;\, ...
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2answers
107 views

example of hom of direct sum

I have a question . Can anyone give me examples for $Hom(B,\oplus A_j)$ not isomorphic to $\oplus Hom(B,A_j)$ or $\prod Hom(B,A_j)$ as abelian groups? Here $A_j$ and B are both modules. I have read ...
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1answer
31 views

Injective Resolution in Abelian Categories

Let $\mathcal{C}$ be an Abelian category. There is a fact that if $\mathcal{C}$ has enough injective objects, then any object in $\mathcal{C}$ has an injective resolution. By the definition of ...
4
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1answer
60 views

A simpler definition of the snake map?

I would like to ask whether the following definition of the connecting morphism in the long exact sequence in homology of a pair $(X,A)$ is correct. First, define relative cycles and boundaries via ...
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0answers
46 views

Tensor product of $C^*$- algebras

We know from the paper of Douglas and Howe (enter link description here) that the commutator ideal $\mathcal{I}$ of $\mathcal{A}(C(T^2))$, the $C^*$-algebra generated by Toeplitz operators with ...
0
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1answer
45 views

Good Pair corollary of the excision theorem

I have problem with understanding the following proof $q_*$ is isomorphism as q is a quotient map and so outside A, it is a homeomorphism implies that $q_*$ induces isomorphism. Given the above ...
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0answers
28 views

A category is locally finitely presented if the relative purity and purity coincide

Let $A$ be a locally finitely presented additive category, $X$ an additive subcategory. A sequence $0\rightarrow A_1\rightarrow A_2\rightarrow A_3\rightarrow 0$ in $A$ is pure exact if it is ...
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1answer
49 views

Ext and extensions

There are two abelian groups up to isomorphism of order $p^2$, where $p$ is a prime. But Ext$(\mathbb{Z}/p,\mathbb{Z}/p)$ is cyclic of order $p$. I can embed $\mathbb{Z}/p$ into $\mathbb{Z}/p^2$ in ...
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0answers
32 views

Some questions about Chain and Homological groups

Consider an oriented complex $K$ and it's chain group; that's the set $C_p(K)$ of it's $p-$chains endowed with the point-wise addition. If $T_1$ and $T_2$ are two triangulation of the same polyhedron ...
3
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1answer
68 views

On a property of split short exact sequences

Let $A_{\bullet}, B_\bullet$ and $C_\bullet$ be three short exact sequences of groups (not necessarily abelian) out of which $A_\bullet$ and $B_\bullet$ are split. Assume that there is again a short ...