Questions about algebraic methods and invariants to study and classify topological spaces: homotopy groups, (co)-homology groups, and beyond.

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2
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2answers
67 views

prove that the identity map $i:S^n\to S^n$ is not nullhomotopic.

Using the fact that: For all $n\in \mathbb{N}$ there is no retraction $r:B^{n+1} \to S^n$, prove that the identity map $i:S^n\to S^n$ is not nullhomotopic. This is a problem in section 56 of Munkres' ...
2
votes
4answers
27 views

$X$ is contained in an annulus containing a circle in the annulus.Show that $\pi_1(X)$ contains a subgroup isomorphic to $\mathbb{Z}$.

In $\mathbb{R^2}$ let $C=\{|x|=2\}$ and $A=\{1<|x|<3\}$, let $X$ be a path connected set such that $C \subset X \subset A$. Show that $\pi_1(X)$ contains a subgroup isomorphic to $\mathbb{Z}$. ...
2
votes
1answer
53 views

How to show the fundamental group of torus is abelian in a homotopic way?

I know the torus is homeomorphic to $S^1 \times S^1$ and the fundamental group is $ \mathbb{Z} \times \mathbb{Z} $, but in the real case, (let the generators of the torus's fundamental group be $a$ ...
1
vote
1answer
42 views

Showing the image of $H^j(X;\mathbb C^\times)$ lies in the torsion subgroup of $H^{j+1}(X;\mathbb Z)$

Let $X$ be a (compact, if necessary) topological space. Then from the short exact sequence of constant sheaves $$ 0 \to \mathbb Z \to \mathbb C \to \mathbb C^\times \to 0 $$ we have a connecting ...
2
votes
1answer
53 views

A technical step in the proof of Atiyah-Bott fixed point formula

From John Roe, Elliptic operators, topology and asymptotic methods, page 135. Here Roe tried to prove that $$ \DeclareMathOperator{\Tr}{Tr} \sum_{q}(-1)^{q} ...
1
vote
0answers
37 views

How to make the orbit space $T/G$ of torus $T$ homeomorphic to the Klein bottle?

Actually it is one of the exercises of Munkres. $G$ is a group of homeomorphisms of the torus having order $2$. How do I get $G$ in order to make $T/G$ homeomorphic to the Klein bottle? Can anybody ...
3
votes
1answer
47 views

A generalization of Jordan curve theorem to connected open sets in the plane

Problem (Fulton's Algebraic Topology: A First Course, Problem 5.23) Let $U\subseteq\mathbb R^2$ be any connected open set in the plane. If $X\subseteq U$ is homeomorphic to $[0,1]$, then ...
2
votes
1answer
34 views

Triangulation Definition Via Cell Partitions

There are two ways of defining a CW-complex. The first is to "inductively build" CW-complexes: you start with 0-cells as your 0-skeleton, attach 1-cells to that to get your 1-skeleton,... and so on. ...
3
votes
1answer
46 views

Cover a sphere by two closed subsets not containing a closed self-antipodal connected subset?

Question (Fulton's Algebraic Topology, A First Course, Problem 4.40) Suppose the sphere $S^2=A\cup B$ where $A,B\subseteq S^2$ are two closed subsets of $S^2$. Is it true that either $A$ or $B$ must ...
1
vote
1answer
21 views

A question regarding simplicial mappings.

Show that give any map $f_0$ from the set of vertices of $\sigma$ to the set of vertices of $\tau$, where $\sigma$ and $\tau$ are simplices, there is a unique simplicial map $f:\sigma\to\tau$ whose ...
2
votes
0answers
40 views

Proving that homotopic maps have the same degree

Let $M, N$ be compact, connected, oriented manifolds. The degree of a map $f:M \rightarrow N$ is defined as the integer $k$ which satisfies $\int_{M} f^{*}\omega = k\int_{N}\omega$. Using the fact ...
1
vote
1answer
42 views

Proposition 0.16 in Hatcher's AT

In the proof of the quoted proposition, it is mentioned that $D^n \times I$ retracts onto $D^n \times \left\{0\right\} \cup \partial D^n \times I$ and an example is given in a figure with $n=2$, which ...
1
vote
1answer
31 views

What is the induced functor of covering spaces to covering groupoids?

I'm reading May's book, 'A Concise Course in Algebraic Topology' and I'm confused about what he means by the induced functor from a covering space. First, here are some helpful/relevant definitions. ...
1
vote
0answers
23 views

Induced group action: homology vs cohomology

Let $M$ be a compact orientable manifold with finitely generated free abelian cohomology groups in even dimensions and $0$ otherwise. Conditions imposed on cohomology clearly imply that $H_i(M) \cong ...
2
votes
2answers
68 views

Homotopy direct limit versus direct limit

Let $X_1\to X_2\to \cdots$ be an infinite sequence of maps. Then, as I understand, the homotopy direct limit of this sequence is the space formed by taking the disjoint union of the $X_i\times I$ and ...
1
vote
0answers
17 views

Dold's proof of equivalence singular and cellular homology

I would like to ask for some help understanding a claim in Dold's proof of the equivalence of cellular and singular homology. The point is that I don't get why $\delta_n=j_*\delta_*$ where: ...
4
votes
1answer
88 views

Cohomology of wedge equals direct sum of cohomologies

I have seen the following fact used somewhere (for example to show that $\mathbb{RP}^3$ is not homotopy equivalent to $S^3\vee\mathbb{RP}^2$): Let $X,Y$ be two path connected pointed spaces such ...
3
votes
1answer
41 views

Does the $\Omega$-spectrum functor send exact triangles to homotopy cofiber sequences?

The functor $\Omega^\infty\colon Spectra\to Spaces$ which takes a spectrum, replaces it by the associated $\Omega$-spectrum and then takes its $0$th space sends exact triangles to homotopy fiber ...
3
votes
1answer
59 views

The nature of isomorphism between fundamental groups with different base points

New to algebraic topology. Munkres (Topology, 2 ed.) in the last paragraph on page 332 says that "If $X$ is path-connected, all the groups $\pi_1(X,x)$ are isomorphic, so it is tempting to try to ...
0
votes
1answer
31 views

homotopy class of maps in terms of homotopy groups of spectra

Given spectra $X$ and $Y$, the set $[X,Y]$ of homotopy classes of maps from $X$ to $Y$ can be endowed with an abelian group structure. Can the group $[X,Y]$ be expressed in terms of the homotopy ...
1
vote
1answer
27 views

Use the universal cover to prove $\gamma * \gamma$ is nullhomotopic if $\gamma$ is a loop in the projective plane

Use the universal cover to prove $\gamma * \gamma$ is nullhomotopic if $\gamma$ is a loop in the projective plane Well, If I was not asked to prove it this way, I could have argued like : ...
1
vote
1answer
26 views

Embedding a space in its cone

Let $X$ be a topological space, and $C(X)= (X \times [0 ,1])/(X \times {1} )$, define $f\colon X \to C(X)$ as $f(x)=[x,t]$ for some fixed $t$ s.t $\ 0\leq t <1$. I have to show this is a ...
2
votes
1answer
123 views

Homology and topological propeties

i have this theorem with it's proof but i don't understand the last part They use this proposition: My question is Why $\varphi^c\cap U_i$ is closed and pairwise disjoint ? where ...
2
votes
1answer
34 views

Proposition 1A.1 in Hatcher's Algebraic Topology

In the proof of the quoted Proposition, we have a connected graph $X$ and a sequence of subgraphs $X_0 \subset X_1 \subset \cdots$ such that $\cup_i X_i$ is both open and closed. Then Hatcher deduces ...
1
vote
1answer
54 views

What's the meaning of this about the cyclic fundamental group?

"You can also get a cyclic group of order p by attaching a disk to a circle by wrapping it around the circle p times (the fact that the fundamental group is Z/pZ follows from Van-Kampen’s theorem). " ...
2
votes
1answer
72 views

Some questions about cellular homology and cohomology

Consider the CW structure on $\mathbb{RP}^n$ given by one cell in every dimension. This gives rise to the cellular complex $C_\bullet(\mathbb{RP}^n)$ which is generated by a single element $c_i$ for ...
7
votes
1answer
129 views

Compact subspace of a covering space

I've been working through Massey's A Basic Course in Algebraic Topology and I've gotten stuck on the following exercise (V.8.4): Let $X$ be a regular topological space, and $(\tilde{X}, p)$ a ...
1
vote
0answers
21 views

Extension to rational and real chains

In the paper on stable commutator length, D. Calegari says that generalized $\operatorname{scl}$ function can be extended by linearity to rational group $1$-chains and by continuity to real chains ...
2
votes
0answers
26 views

Euler Classes, Chern Classes, $S^2$ Bundles, and $CP^1$ Bundles

I am just starting out learning about characteristic classes (Euler, Chern, etc.) from Bott and Tu's book, and I had the following question. Let $E$ be an oriented $S^2$ bundle over $M$ with ...
1
vote
1answer
38 views

Does the Wirtinger presentation extend to compliments of graphs and links?

In a previous question I asked about a specific fundamental group problem, which was resolved via SVK but I was also interested in whether or not the Wirtinger presentation was valid in some way. In ...
1
vote
1answer
51 views

A continous map between the two torus and the torus

Let $\Sigma$ be the doubled torus (a compact oriented) surface of genus 2) and let $T$ be the torus. Suppose $f: \Sigma \rightarrow T$. Prove that $f$ is not a local homeomorphism. Attempt at ...
4
votes
0answers
99 views

Surgery presentations and gluing

It is well known that every closed oriented 3dimensional manifold can be obtained from a framed link in $S^3$. Let $M$ and $N$ be topological 3-manifolds with boundary such that boundaries $\partial ...
0
votes
0answers
60 views

Could anyone help me with the hatcher algebraic topology 1.3.6 [closed]

It tells me to use the space shown in the picture as the covering space of shrinking wedge of circles. But I can't see how to define the covering map from this space to the wedge of circles. Can ...
4
votes
1answer
59 views

Mayer-Vietoris where $A\cap B$ bounds $A$ and $B$

So I'm a bit confused about how the Mayer-Vietoris Sequence works. I thought that one of the times when it is useful is when we choose $A$ and $B$ such that $A\cap B$ is homotopic to the boundaries of ...
2
votes
3answers
108 views

Fundamental group of complement of circle with a line passing through it.

Let $X= S^1 \cup y$-axis in the $xy$-plane. Compute the fundamental group of $\mathbb{R}^3 - X$ Edit: To clarify, $S^1 =\{(x,y)|x^2 + y^2 = 1\}$ is the unit circle in the $xy$-plane, so that the ...
0
votes
0answers
32 views

A curve not homotopic to constant path but index of every point is zero.

I want to find a curve which is not homotopic to constant path but the index of every point not on the curve is zero. Here the domain is an open subset of Complex Plane.I was unable to find any such ...
5
votes
0answers
112 views

Learning roadmap to Topological Quantum Field Theories from a mathematics perspective

I want to learn TQFT's and am looking for review articles or books. My mathematics knowledge is limited to one year of graduate course in Algebra (Groups,Rings,Fields,Categories, Modules and ...
2
votes
1answer
53 views

Rank of $H_1(X)$ for a retract of $M_g$

I'm trying to solve the following exercise and I would be grateful if I could get a hint: If the closed orientable surface $M_g$ of genus $g$ retracts onto a graph $X\subset M_g$, then the rank of ...
3
votes
1answer
95 views

Homology of 5-manifold

Let $M$ be a closed connected 5-manifold such that $\pi_1(M)=\mathbb{Z}_7$ and $H_2(M)=\mathbb{Z}^2$. I would like to compute its homology and cohomology. Standard computations involving Poincaré ...
0
votes
2answers
46 views

Why is this a quotient map

Is there a direct way to see that $p \times id : [0,1]^2 \rightarrow S^1 \times [0,1]$ is a quotient map with $(p \times id)(x,y) = (e^{ix},y)$? By direct way, I mean is there an obvious argument why ...
1
vote
2answers
59 views

what is the fundamental group of a torus with $k$ points removed

I was about to use the Seifert-van Kampen to compute the fundamental group of a torus of genus 2. In the process, I need to know the fundamental group of a torus (of genus one) with a hole removed. is ...
1
vote
1answer
30 views

Homology computation

I was computing the homology of a space obtained from 2 n-spheres $S_1^n,S_2^n$ by identifying them along their equatorial $(n-1)$-spheres. I had a doubt but I figured it out while typing, so I'm ...
4
votes
0answers
129 views

Standard spaces with non-standard topology and around

The following series of questions gives me no rest. Let $\mathbb{\widetilde{R}}^n$ be $\mathbb{R}^n$ with Zariski topology, i.e. we say that $A\subset \mathbb{R}^n$ is closed if it is given by the ...
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votes
0answers
84 views

Question about Homology from the Chang's book: Methods in Nonlinear analysis

In the K.C.Chang's book in page $336$ of the book this corollary without prove there is a theorem before it but I don't know if it is a corollary of this theorem, how I can prove this ...
3
votes
0answers
36 views

Brief summary of simplicial, CW and manifold notions

I tried to summarize the relations between the following notions of: a manifold (smooth, topological and PL), simpilicial complex, CW complex. However I found some inconsistencies, which may be not a ...
2
votes
1answer
37 views

Extending a homeomorphism of the open disk to the boundary.

Let $D^2 = \{x \in \mathbb{R}^2 : ||x||\leq 1\}$ denote the closed disk and $int(D^2)$ denote its interior. If I have a homeomorphism $\ f: int(D^2) \rightarrow int(D^2)$ it is clear that it is not ...
2
votes
1answer
36 views

K-theory product trivial on $K^1$ due to stability?

Let $K^0$ be the topological complex vector bundle $K$-theory functor. On the one hand, we have that the product on $K^0(\Sigma X)$ vanishes by a Mayer-Vietoris argument. On the other hand, we define ...
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vote
0answers
28 views

References for the moduli space of conformal structures on a disk minus boundary points

In an article I'm reading, it is said that the moduli space of conformal structures on a disk minus $k+1$ boundary points is a ($k-2$)-dimensional manifold. I want to understand why and have done some ...
3
votes
1answer
41 views

proving that two paths are homotopic

Let $X$ and $Y$ be path connected and let $h : X → Y$ be a continuous function which induces the trivial homomorphism of fundamental groups. Let $x_0, x_1 ∈ X$ and let $f$ and $g$ be paths from $x_0$ ...
1
vote
3answers
119 views

Some practical questions on cohomology and the ring $\mathbf{Z}[x]/(x^2)$

So I know that the cohomology ring of $S^n$ is $\mathbf{Z}[x]/(x^2)$ with "$x$ in degree $n$"; if we ignore the grading then this ring fails to distinguish the spheres. What is actually meant by "in ...