Questions tagged [cohomology-operations]

Use this tag for questions about natural transformations from a functor defining a cohomology theory to itself. Common examples include Steenrod squares in mod 2 cohomology and Adams operations in K-theory.

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1answer
30 views

On how to compute $H_*(\Omega^2 S^{n+2}, \mathbb Z/2)$ and its Steenrod operations

I've been trying to understand the calculation of $H_*(\Omega^2 S^{n+2},\mathbb Z/2)$ as an algebra, together with the dual Steenrod operations on it which lower the degrees. I've spent many hours ...
5
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1answer
48 views

Which integral Steifel-Whitney classes are universally $0$?

Let $BO(n)$ denote the classifying space of the orthogonal group $O(n)$. Then there is the well-known ring isomorphism $$H^*(BO(n);\mathbb{Z}/2) \cong \mathbb{Z}/2[w_1,\dots,w_n] $$ where $w_i \in H^...
4
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1answer
61 views

Is it possible to find a torsion-free space with non-trivial $\operatorname{Sq}^{2^n}$

Is it possible to find a torsion-free space with non-trivial $\operatorname{Sq}^{2^n}$ ? Torsion-free means integral cohomology is torsion-free. When $n \leq 3$, such spaces are projective space $\...
5
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0answers
136 views

Why is the Bockstein morphism a derivation?

I'm trying to understand the Bockstein morphism in cohomology, and one of the points is that $\delta : H^*(G,\mathbb{F}_p)\to H^*(G,\mathbb{F}_p)$ is a derivation that squares to $0$. I could ...
1
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1answer
126 views

Group homomorphism induces cohomology homomorphism

Let $\phi:G\to H$ a group homomorphism. I'm interested in knowing what can be said about $H^*(X;G)$ and $H^*(X;H)$ (singular cohomology) in terms of $\phi$. One can define $\phi^*:C^n(X;G)\to C^n(X;...
1
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1answer
85 views

Reference request: The Steenrod squares generate all stable cohomology operations

The isomorphy of the Steenrod algebra to the cohomology of an Eilenberg-MacLane spectrum is a direct corollary of Brown representability and the fact, that all stable cohomology operations are ...
5
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2answers
370 views

How can I determine the Steenrod Square $Sq^2$ for complex projective space?

I am trying to learn about Steenrod Squares for algebraic varieties so that I can compute examples of complex topological K-theory using the Atiyah-Hirzebruch Spectral Sequence (AHSS). One of the key ...
6
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2answers
397 views

Intuition behind cohomology operations

I try to unterstand cohomology operations, but I cannot get the intuition behind it. Could someone explain the intuition behind it? My background: I have a basic understanding of homology and ...
2
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1answer
254 views

Applications of Steenrod Squares

I am reading through Glen Bredon's book "Geometry and Topology" and after giving the axioms for Steenrod squares plus the Adem relation, he proves that if $i$ is not a power of $2$ then $Sq^i$ is ...
4
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1answer
271 views

Closed manifolds with isomorphic cohomology rings, but different cohomology modules over the Steenrod algebra

For any $n > 2$, $\mathbb{CP}^n/\mathbb{CP}^{n-2}$ and $S^{2n}\vee S^{2n-2}$ have the same cohomology groups: for any ring $R$, we have $$H^k(\mathbb{CP}^n/\mathbb{CP}^{n-2}; R) \cong H^k(S^{2n}\...
3
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1answer
86 views

Some clarifications about the Secondary Cohomology Operation associated to $Sq^2\circ Sq^2=0$

As explained in the title, I'm looking for some clarifications about the secondary cohomology operation associated to the relation $Sq^2\circ Sq^2=0$. I've just started reading the relevant chapter in ...
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0answers
68 views

Expositions of Postkinov Towers

I am giving a lecture on Postkinov towers and I want to teach the students a lesson :). These students have seen local coefficients, spectral sequences and cohomology operations at the level of ...
5
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0answers
58 views

Characterisation of Stable Cohomological Operations: $\Sigma (\tau_n (\imath_{A,n}))=\rho_{n+1}^*(\tau_{n+1}(\imath_{A,n+1}))$

I've started studying (stable) cohomological operations on my lecture notes, and I was given that an equivalent definition for a family of cohomological operations to be a stable cohomological ...
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2answers
475 views

Reference for secondary cohomology operations

I am learning some homotopy theory and am currently reading Mosher and Tangora. I love the content of this book, it's very terse and comes straight to the point. At the same time I find it very ...
3
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1answer
172 views

The smallest $n> 0$ with the nonzero $n$th Stiefel-Whitney class is a power of 2 when total Stiefel-Whitney class is not trivial.

This is the Problem 8-B form the characteristic classes by John W. Milnor and James D. Stasheff. [Problem 8-B]. If the total Stiefel-Whitney class $w(\xi) \neq 1$, show that the smallest $n>0$ ...
6
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2answers
1k views

Calculate the Wu class from the Stiefel-Whitney class

The total Stiefel-Whitney class $w=1+w_1+w_2+\cdots$ is related to the total Wu class $u=1+u_1+u_2+\cdots$: The total Stiefel-Whitney class $w$ is the Steenrod square of the Wu class $u$: \begin{align}...
3
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1answer
118 views

differential in AHSS for spin cobordism

According to these solutions, the differential $d_2: H_p(X,\Omega_1^{Spin})\rightarrow H_{p-2}(X,\Omega_2^{Spin})$ is the dual of $Sq^2$. Why? This MO post asks a similar question (but about $d_3$ in ...
11
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1answer
282 views

Steenrod Algebra as automorphisms of additive group

Is there a direct way to see that the subalgebra of the mod-$p$ Steenrod algebra ${\mathcal A}_p$ generated by the reduced powers is isomorphic to the dual of the Hopf algebra ${\mathcal O}(\text{Aut}(...
6
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1answer
229 views

Why does «Massey cube» of an odd element lie in 3-torsion?

The cup product is supercommutative, i.e the supercommutator $[-,-]$ is trivial at the cohomology level — but not at the cochain level, which allows one to produce various cohomology operations. The ...
25
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1answer
1k views

The simplest nontrivial (unstable) integral cohomology operation

By an integral cohomology operation I mean a natural transformation $H^i(X, \mathbb{Z}) \times H^j(X, \mathbb{Z}) \times ... \to H^k(X, \mathbb{Z})$, where we restrict $X$ to some nice category of ...
6
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1answer
132 views

Prove that Steenrod squares are stable

I have been studying the mod 2 Steenrod algebra. And I try to solve some exercises of it. Can you help me to check this proof: Let $SX$ denote the suspension of $X$, and let $S: \underline{H}^q(X) ...
16
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2answers
3k views

The “Wu formula” and Steenrod algebras

The Wikipedia page on Stiefel-Whitney classes includes the following paragraph: Over the Steenrod algebra, the Stiefel–Whitney classes of a smooth manifold (defined as the Stiefel–Whitney classes ...
0
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1answer
158 views

Steenrod squares on the sphere

Let $S^d$ denote the $d$-sphere. The only non-trivial cohomology groups are $H^0(S^d;\mathbb Z_2)= \mathbb Z_2$ generated by $1$ and $H^d(S^d;\mathbb Z_2)= \mathbb Z_2$ generated by the fundamental ...
10
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653 views

Steenrod Algebra - Converting between Milnor to Serre-Cartan basis'

I have been studying the mod 2 Steenrod Algebra (denoted $\mathcal{A}$), using Mosher & Tangora. We have the Serre-Cartan (or Adem basis): Let $I = \{i_1,i_2,\ldots,i_n\}$ be a sequence of ...
10
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0answers
279 views

The action of the Steenrod algebra on $H^*(BU; \mathbb{Z}_p)$

By considering the classifying map $f \colon (\mathbb{C} P^{\infty})^n \rightarrow BU(n)$, its induced map on cohomology, and using the Cartan formula, we can derive the Wu formula for the action of ...