This tag is for questions relating to cohomology groups and cochain complexes.

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1answer
67 views

Decomposition of cohomology group on $S^{n}$

If we have decomposition of cohomology group on $S^{n}$ it looks like $H^{n}(S^{n})=H^{n}(S^{n})_{+}\oplus H^{n}(S^{n})_{-}$, where $H^{n}(S^{n})_{\pm}$ cohomology of invariant or anti-invariant $n$ ...
9
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1answer
208 views

Relative de Rham Cohomology is Homotopy Invariant

Suppose $ f:N\rightarrow M$ is a smooth map between two manifolds. Relative de Rham cohomology is defined through the complex $ \Omega^{q}(f)=\Omega^{q}(M)\oplus\Omega^{q-1}(N)$ with ...
3
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1answer
45 views

Well-definedness of a coboundary map between a reduced $L^2$ de Rham cohomology group and a relative cohomology group

I'm working right now with this paper of Carron. And I think I'm stuck at a relatively simple question. On page 11 he is defining a coboundary map $b : H^k_{2, \text{reduced}}(M - K) \to H^{k+1}(K, ...
2
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1answer
75 views

Poincare lemma via Lie derivative

I found such a beautiful proof of Poincaré lemma here (in Russian): Let $B$ be a star-like neighborhood of 0 in $\mathbb{R}^n$ and $r=\sum x^i \frac{\partial}{\partial x^i}$. Then Lie derivative ...
2
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1answer
40 views

Wedge product of basis elements of cohomology

Let $M$ be a compact, connected, oriented 4-manifold without boundary. If $H^2(M)\cong \mathbb{R}^2$ and I have a basis $\{[\omega_1],[\omega_2]\}$ for $H^2(M)$, is it the case that $[\omega_1\wedge ...
2
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2answers
52 views

Recommended textbooks for Hamiltonian group actions?

I am doing a project on Hamiltonian group actions on symplectic manifolds, and my supervisor was able to list several good books on Riemannian geometry to start me off, but he didn't know of any ...
2
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1answer
74 views

De Rham cohomology for $\mathbb{R^2}$

De Rham cohomology groups for $\mathbb{R^2}$. $H^{0}_{dR}(\mathbb{R}^{2})=\mathbb{R}$ since $Z^{0}(\mathbb{R}^{2})$ is the one dimensional space of locally constant functions on $\mathbb{R}^{2}$ and ...
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1answer
45 views

Prove that $H^{2}(S^{2})\neq 0$

Prove that $H^{2}(S^{2})\neq 0$ Suppose $\omega$ is an area form of $S^{2}$. An arbitrary two form on $S^{2}$ is closed as if $f(x,y)dx\wedge dy\in\Omega^{2}(S^{2})$ then $d(f(x,y)dx\wedge dy)=0$. I ...
2
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2answers
87 views

Total dimension of the cohomology of a homogeneous space (or of a graded Tor)

I want to calculate the cohomology ring with rational coefficients of a homogeneous space, but would be happy enough to know its total dimension. Let $G$ be a compact Lie group, $T$ a maximal torus, ...
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0answers
50 views

Intuition of higher push-forward constant sheaves.

Let us consider the higher phsh-forward sheaves $R^if_*\mathbb{R}$ of a map $f:X\rightarrow Y$ between two compact manifolds. We assume that the fibers has a constant dimension, say $n$. I think ...
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0answers
34 views

Are acyclic coverings cofinal in the set of coverings?

I am interested by the following question in algebraic geometry. Recall that a covering $\mathfrak{U}$ of a topological space $X$ is acyclic for a sheaf $\mathscr{F}$ if we have $H^q(U_{i_0,\cdots, ...
2
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2answers
51 views

What is the point of triangulating topological spaces?

In a general sense, what is the purpose to triangulating, for example, a 3-dimensional topological space? What advantages does it give if we can triangulate a Seifert-Weber space into 23 tetrahedra? ...
2
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1answer
47 views

Hecke equivariance in Poincaré duality.

Consider the first singular homology and cohomology groups of a modular curve, $H^1(X,\mathbb{Z})$ and $H_1(X,\mathbb{Z})$. The Hecke algebra acts on both of them and they are dual to each other under ...
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0answers
62 views

In the Universal Coefficient Theorem, how does the cohomology generator relate to the homology generators?

Consider homology and cohomology of some space $X$ where the homology groups are finitely generated. Consider $tor(H^i(X))$, the torsion part of $H^i(X)$. How do the generators of $tor(H^i(X))$ ...
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0answers
38 views

To prove Pic$(X)$ is isomorphic to $H^1(\mathcal{U},O_X^*)$

We start with an invertible sheaf $\mathcal{L}$ and an open cover $\mathcal{U}=\{U_i\}_i$ for $X$, such that $\mathcal{L}|_{U_i}=O_X|_{U_i}$. So the line bundle is given by the information : ...
0
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2answers
144 views

Cohomology of twice-punctured plane

I am looking for pure calculus type of solution for computing the cohomology class $H^2$ of $R^2-\{p , q\}$ ie plane with two points removed. *I was not precise I apologize: Every closed 2-form is ...
3
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1answer
58 views

Why do we have that Hom is an exact functor in the situation described below?

We are given a finite $p$-group $G$ and a finite $G$-module $M$ such that $pM=0$ (therefore $M$ is in particular a $\mathbb{F}_p$-vector space). In addition we have an arbitrary $G$-module $N$ which ...
5
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0answers
62 views

There does not exist a map $S^2\times S^2\to \mathbb{CP}^2$ with odd degree.

The following is a problem from a topology qualifying exam I am studying for: Show there does not exist a map $S^2\times S^2\to \mathbb{CP}^2$ with odd degree. I think I am doing something wrong, ...
3
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0answers
51 views

For which categories do injections induce surjections in cohomology?

I'll ask a specific question first, but I believe my question might have a rather immediate abstraction, with which I'll finish. Let $H,G$ be finite affine group schemes over an algebraically closed ...
3
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0answers
55 views

The vanishing (?) cohomology of the Milnor fiber

Setup. Say we have a germ of a holomorphic function $f:(\mathbb C^{n+1},0)\to (\mathbb C,0)$ with a critical point at the origin. There is an $\epsilon>0$ small enough so that $f$ becomes a ...
6
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1answer
90 views

Cohomology of $S^2\times S^2/\mathbb{Z}_2$

The product of two spheres admits a diagonal $\mathbb{Z}_2$-action, $(x,y)\mapsto (-x,-y)$. I'm trying to compute the integral singular cohomology ring of the orbit space $X$ of this action. $X$ is ...
0
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1answer
31 views

mapping in $H_{c}^k(X,\mathbb{Z})$

If Y is closed subset of X, do anyone know what is the mapping from $H_{c}^k(X,\mathbb{Z})$ to $H_{c}^k(Y,\mathbb{Z})$ ? Is it differ if X is an open subset of Y. $H_{c}^k$ is a cohomology with ...
4
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1answer
74 views

Examples of de Rham cohomology being easier to compute that singular cohomology

De Rham's theorem states that for any smooth manifold $M$ the singular cohomology and de Rham cohomology of $M$ are isomorphic. Are there any examples of manifolds for which it is easier to compute ...
2
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1answer
129 views

De Rham cohomology, and forms on manifolds

In String Theory and M-Theory by Becker, Becker and Schwarz, they introduce a group, $$C^{p}(M)$$ which they denote the group of all closed $p$-forms on the manifold $M$. Furthermore, they state ...
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146 views

Compactly supported Dolbeault Cohomology: is this True?

nLab states that for $D$ the unit disk in $\mathbb C$, the cohomology of the complex $$ (\Omega_c^{1,\ast}(D),\overline{\partial})$$ is the continuous dual of the space of holomorphic functions ...
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1answer
31 views

Why is the generator of $H^0(S^n,\mathbb{Z})$ the identity of $H^*(S^n,\mathbb{Z})$?

I know the cohomology ring $H^*(S^n,\mathbb{Z})\simeq\mathbb{Z}[X]/(X^2)$, but the computation of this uses that fact that a generator of $H^0(S^n,\mathbb{Z})$ is the identity for the cohomology ring ...
2
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1answer
53 views

Computing $H^\bullet(\Bbb Z/n\Bbb Z)$

This is related to this other question of mine Showing that $\operatorname {Br}(\Bbb F_q)=0$ in which I also got stuck at writing a free resolution. I want to compute the group cohomology ...
5
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2answers
87 views

Showing that $\operatorname {Br}(\Bbb F_q)=0$

I want to prove that $\operatorname {Br}(\Bbb F_q)=0$ using the cohomological description of the Brauer group. We have: $\operatorname {Br}(\Bbb F_q)=H^2(\operatorname {Gal}(\overline {\Bbb ...
4
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1answer
146 views

Thom–Gysin long exact sequence

I have read about the following exact sequence of cohomology: Let $V$ be an algebraic variety over $\mathbb{C}$. If $U\subset V$ is an open subvariety, then there is a long exact sequence for ...
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0answers
84 views

Long Exact Sequence Cohomology with Compact Support

I found in this topic or (question) a reason for my question, but i do not understand it. As this question is quite old, I hope someone else can help me. Assume $U$ is an open subset of a topological ...
2
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0answers
43 views

Cohomology-Homology bilinear form of Seifert surfaces

Let $C_\ast$ be any chain complex of $R$-modules. Then for any $k\in\mathbb{Z}$ we obtain a $R$-bilinear map $$\langle-,-\rangle:H^k\!C_\ast\times H_kC_\ast\longrightarrow R, ...
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1answer
40 views

On the hypothesis of the Additive Cousin Problem

The Additive Cousin Problem is the following: Assume that $D$ is a region (open, connected) of $\mathbb{C}^n$. Assume that the Dolbeault Cohomology Group of $D$, $H^1_{\bar{\partial}}(D)$ is equal to ...
11
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2answers
150 views

Computing cohomology of hypersurface

I'm taking a course on differential geometry now, and we got the following exercise from the lecturer: compute the (de Rham) cohomology groups $H_{dR}^i(M)$ of your favourite space. In all the ...
4
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1answer
64 views

Can the cohomology vanish on $X$ but on no restriction to hyperplane sections?

I would need a result asserting that if, say, a locally free sheaf $\mathcal{F}$ on a projective $X$ has non-vanishing cohomology when restricted to any smooth hyperplane section, then $H^1(X, ...
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0answers
13 views

How to Resolve Extension Issues in Equivariant (Co)Homology Computations

I am computing equivariant homology, which is just the usual homology of the Borel construction. I have reached a few extension issues. I have used the Gysin sequence, Leray-Serre spectral sequence, ...
4
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0answers
100 views

Eilenberg-Moore Spectral Sequence for Homology with Coefficients in the Integers

I am trying to learn about the Eilenberg-Moore spectral sequence to compute homology and cohomology. I have been using Hatcher's book on spectral sequences and also McCleary's "A User's Guide to ...
2
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1answer
76 views

show that the compactly supported De Rham cohomology groups $H^{p}_{DR}(\mathbb{R^n})$ are all zero for $0\leq p<n$

I am reading John Lee's book and there is a problem on De Rham cohomology: show that the compactly supported De Rham cohomology groups $H^{p}_{DR}(\mathbb{R^n})$ are all zero for $0\leq p<n$. My ...
1
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1answer
37 views

Vanishing of local cohomology groups

Let $k$ be a field and let $X$ be a smooth separated $k$-variety. Let $T$ be a closed integral subscheme of $X$ of generic point $\eta$. The object of interest here is the local cohomology group $$ ...
4
votes
1answer
202 views

Universal coefficient theorem with ring coefficients

The universal coefficient theorem for cohomology reads: $$0 \to Ext(H_{n-1}(C), R) \to H^n(C;R) \to Hom(H_n(C), R) \to 0,$$ where $C$ is a chain complex of free abelian groups and $R$ is a ring. It ...
2
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1answer
56 views

Is there any simple example that $lim^1$ terms appear?

limit of cohomology does not behave well in the sense that there will be $lim^1$ term. Is there any simple example that $lim^1$ terms appear? Thanks!
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3answers
116 views

Cohomology groups of real projective space

My question concerns the cohomology groups $H^k(RP^n,\mathbb{Z}_2)$. We know that $H_k(RP^n,\mathbb{Z}_2) = \mathbb{Z}_2$ if $0 \leq k \leq n$ and is trivial otherwise. I looked up the solution and it ...
6
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1answer
63 views

Image of the Brauer group under a field extension

For $k$ a field, let $Br(k)$ - the Brauer group of $k$ - denote the group of finite-dimensional central simple algebras over $k$, modulo Morita equivalence $(A\equiv B\iff \exists m, n(A\otimes_k ...
1
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1answer
41 views

Homology/Cohomology of Closed Manfold with $\mathbb{Z}_{2}$ Coefficients

Why is $H_{i}(M,\mathbb{Z}_{2}) = H^{i}(M,\mathbb{Z}_{2})$ for a closed manifold $M$? (Hatcher states this on p. 249 in his proof of Corollary 3.37.) Thanks.
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1answer
36 views

$\tilde{H}^i(\sum X) \cong \tilde{H}^{i-1}(X)$

I need help with this problem Let $\sum X =C^+X \cup_X C^-X$ be the union of two cones on $X$ with a common base. Show that $\tilde{H}^i(\sum X) \cong \tilde{H}^{i-1}(X)$. Dose this give an ...
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0answers
11 views

Functional isomorphism between $Hom_{\mathbb{Z}}(C,A)$ and $Hom_{\Lambda}(C \otimes \Lambda, A)$

In the proof of the following lemma: Let $G$ be a finite group, and let $A$ be an injective $\mathbb{Z}[G]-module$. Then $A$ is $\mathbb{Z}-injective$, i.e. divisible. It defines $\Lambda=Z[G]$, and ...
1
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1answer
47 views

Natural map of extension groups

Let $\Lambda$ be a cocommutative Hopf algebra over a commutative ring $R$. For two left $\Lambda$-modules $M$ and $N$, interpret $\mathrm{Ext}_{\Lambda}^n(M,N)$ as the set of equivalence classes of ...
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0answers
30 views

What is explicit isomorphism between $H^2(G,\mathbb{Q}/\mathbb{Z})$ and $H_2(G,\mathbb{Z})$?

Let $G$ be a finite group. Then its Schur multiplier is the second cohomology group $H^2(G,\mathbb{Q}/\mathbb{Z})$, which is isomorphic to the second homology group $H_2(G,\mathbb{Z})$ (Proof can be ...
2
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1answer
156 views

Definition/existence/uniqueness of a minimal projective resolution

I'm reading Dave Benson's book "Representations and Cohomology," Volume I, and I'm trying to understand the following discussion on page $32$ in which he introduces the notion of a minimal projective ...
2
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1answer
114 views

Understanding the Bockstein homomorphism in group cohomology

Let $k$ be an algebraically closed field of characteristic $p>0$. Let $G$ be a finite group. In group cohomology, the Bockstein homomorphism is the connecting homomorphism ...
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1answer
84 views

Cohomology of the structure sheaf of $\mathbb{P}^1 \times \mathbb{P}^1$

I need to compute the euler characteristic $\chi(\mathcal{O}_{\mathbb{P}^1 \times \mathbb{P}^1})$ of the structure sheaf of $\mathbb{P}^1 \times \mathbb{P}^1$ (the product of the projective complex ...