# Tagged Questions

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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### Exact Sequences in algebraic geometry [closed]

A very basic question. I am going to take my first course in Algebraic Geometry next semester and I am now repeating some commutative algebra to be prepared. I just came up to the part of Homological ...
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### A map of complexes which is zero on cohomology but not zero in $D(\mathcal{A})$

Yesterday I asked a very similar question about an exercise of Gelfand's book "Methods of Homological Algebra". In the comments it was pointed out that there was an easier version of that exercise but ...
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### Exercise from “Methods of Homological Algebra” Gelfand

I have to show that a map of complexes $f: A^{\bullet} \to B^{\bullet}$ in $Ab$ with $H^{n}(f)=0$ is not necessarily 0 in the derived category $D(\mathcal{A})$. To find this counterexample I'm given ...
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### Filtered Colimit of associative $k$-algebras that are domains

Let $C$ be a filtered subcategory of the category of commutative algebras over a fixed field $k$ whose objects are all integral domains. Then the colimit of the obvious diagram is an integral domain. ...
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### The cohomology of the Dirac operator $d+d^{*}$

Let $(M,g))$ be a Riemannian manifold with the Hodge dual operator $d^{*}$. Is there a name (and some computation in some reference) for the cohomology of the complex of Harmonic forms with ...
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### Chain complex and free resolution

If $I \subset R = k[x_1,\dots,x_n]$ is an ideal. Then why: $0 \to C_i \to \dots\to C_0 \to R \to R/I \to0$ is a free resolution of $R/I$ if and only if $0 \to C_i \to \dots\to C_0 \to I \to 0$ is a ...
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### Regular sequence in degree 1

$R$ is a graded algebra generated by $R_1$(the degree 1 piece) over $R_0=k$ where $k$ is a infinite field and R has no negative degree. Given irrelevant ideal has depth d, then is it possible to find ...
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### Motivation for the mapping cone complexes

I was reading some topics in Homological Algebra when I came across the concepts of cone of a map of complexes and cylinder. My knowledge of Algebraic Topology is pretty basic so I only used these ...
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### Eilenberg–Zilber as abstract nonsense - why is it important?

The Eilenberg–Zilber theorem in singular homology, relating the monoidal structure of the category of chain complexes with the chain complex of the cartesian product of the underlying spaces, is used ...
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### Need Counterexample to show Koszul complex is not minimal free resolution?

Recall that Koszul Complex $K.(f,g)$ of polynomials $f,g \in k[x_1,\ldots,x_n]=:R$ is defined as:$$0 \to R \overset{\phi_1} \to R^2 \overset{\phi_2} \to R \to 0$$ where $\phi_1$ and $\phi_2$ are ...
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### Tensoring a connective chain complex with a simplicial set

Let $\mathrm{Ch}_{\geq 0}(R)$ be the category of chain complexes of $R$-modules concentrated in nonnegative degrees, equipped with the projective model structure. By a general theorem about model ...
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### Short exact sequence of groups, is it possible to construct an associated fibration of spaces?

Let$$0 \to R \to F \to G \to 0$$be a short exact sequence of groups. Is it possible to construct an associated fibration of spaces$$K(R, 1) \to K(F, 1) \to K(G, 1)?$$
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### Universal Property of free objects

I am working on free objects, I am restricting myself primarely to groups, rings and modules (with maybe algebras) so in a sense in the concrete category (if I am not mistaken. This is a thesis work I ...
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### Show that the $i$th local cohomology functor is zero for $i > 0$

Let $I$ be an ideal of a Noetherian ring $R$, and let $M$ be a module over $R$. Let $\Gamma_I(M)$ be the set of all elements $m$ of $M$ for which $I^n m = 0$ for some $n \geq 1$. Then $\Gamma_I(-)$ ...
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### Errata in Prof. Rotman AIHA book about projectives in the chain complex category (section 10.5)

EDIT After thinking carefully with the help of the clear answer of ZhenLin, I think I will reformulate my question the following way. The text of my original question is kept below. The claim of Prof....
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### Global dimension of matrix algebra

I want to calculate the global dimension of this algebra.  \quad A = \begin{pmatrix} k & 0 & 0& 0 \\ k & k & 0&0\\ k&0&k&0\\ k&k&k&k \end{pmatrix} \...
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### Lifting the projective property through the affine centre

Let $\mathbb{k}$ be an algebraically closed field. There are many interesting examples of $\mathbb{k}$-algebras $R$ which admit a large central subalgebra $Z_0$ such that $R$ is a free $Z_0$-module ...
Let $E^\bullet$ be a perfect complex of $R$-modules (where $R$ comm. ring). So $E^\bullet$ is quasi-isomorphic to a bounded complex of finitely generated projective R-modules. Now $E^\bullet$ has Tor-...
Let $G$ be a finite group, although this may not be necessary for almost everything that follows. One of the ways of defining Galois homology groups is using the standard resolution for the \$\mathbb{Z}...