A topological group is a group endowed with a topology such that the group operation and inversion are continuous maps. They are useful in various areas of mathematics. Every topological vector space is a topological group. Locally compact groups are important in harmonic analysis.

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30 views

True or False: Topological Group and $S^1 \vee S^1$

$i.$ $S^1 \vee S^1$ can be embedded in a topological group $ii.$ $S^1 \vee S^1$ can be covered by a topological group I think $i.$ is true since we can embed the wedge sum into $\mathbb{R}^2$, which ...
0
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0answers
11 views

Equivalent definitions of a lattice in a real vector space of finite dimension

I'm currently trying to work my way through chapter seven of Serre's book "A Course in Arithmetic" with a view to learning about modular forms. During the course of this chapter the book begins to ...
0
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1answer
16 views

Closed kernel in a compact group is open

The way I think it should work is that $${\rm ker} = \bigcap_{g \notin {\rm ker}} (G - g\,{\rm ker}),$$ with each $G - g\,{\rm ker}$ open. Since $G$ is compact, there should, in fact, only be ...
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0answers
42 views

Family of equivalent unitary representations is not a set.

I have recently come across a statement in the book: Kazhdan's property (T) by B. Bekka, P. de la Harpe, A. Valette at the beginning Appendix F.2. Fell topology on sets of unitary representations. ...
1
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1answer
36 views

If A has positive Haar measure then $AA^{-1}$ is a neighborhood of $e$

I read the following exercise: Prove that if $G$ is a locally compact topological group with Haar measure $\mu$ and $A \subset G, \mu (A) >0$, then $AA^{-1}$ contains an open neighborhood of the ...
5
votes
1answer
41 views

Characters of a Group: two definitions

If $G$ is an abelian group, the characters associated to the rapresentations of $G$ over $\textrm{GL}_1(\mathbb C)=\mathbb C^\ast$ are simply the group homomorphisms: $$\chi:G\longrightarrow\mathbb ...
0
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1answer
21 views

Product of compact subsets of a topological group is compact

I need to show that if $G$ is a topological group and $A,B\subseteq G$ are compact subsets, then $AB$ is compact. Thanks
2
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0answers
33 views

Dense subgroup of an extremely amenble group is extremely amenable??

Recall that a topological group $G$ is called extremely amenable if every continuous action of $G$ on a compact space $K$ admit a fixed point. i.e there is a point $\xi\in K$ such that $g.\xi=\xi$ for ...
1
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1answer
49 views

Topological group with discrete topology

Let $G$ be a topological group. I came to know that if I can show the existence of a homeomorphism of $G$ which moves only finitely many points of $G$, then $G$ has only discrete topology. How can I ...
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2answers
82 views
+50

Creating a Topological group from modulo multiplication Group.

If I were to create a Topology out of the Modulo 3 Multiplication group $\mathbb{Z}_3$, what elements would it consist of and why? So $\mathbb{Z}_3 = \{0,1,2\}$ as a group over modulo 3. What are the ...
1
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0answers
70 views

Closed and Connected subgroups of $\mathbb{R}^n$

Question is : What are closed connected subgroups of $\mathbb{R}$ and from that deduce what are closed connected subgroups of $\mathbb{R}^n$ What i have done so far is : Only connected subsets of ...
0
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0answers
29 views

Can a compact topological group have the trivial topology?

I want to show that the following are equivalent for a compact topological group $G$: $G$ is the inverse limit of finite groups $G_i$. There's a family $\left\{N_i\right\}$ of open normal subgroups ...
-1
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1answer
66 views

Locally compact groups

Let $S= G_1\bigcup G_2 $, where $G_1$ and $G_2$ are two groups. If $S$ is locally compact, is it true that either $G_1$ or $G_2$ is locally compact? Thank you.
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0answers
55 views

Any topological group is Hausdorff if $\{1\}$ is closed.

Let $G$ be a topological group and $\{1\}$ is closed where $1$ is the identity element of $G$. Then for any two distinct element $x,y \in G$ , we have an open neighbourhood $U$ of $x$ not containing ...
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1answer
36 views

Is every compact monothetic group metrizable?

If $G$ is a compact (Hausdorff) topological group with a dense subgroup $H \cong \mathbb Z$, is it necessarily true that $G$ is first countable? This claim seems to be implicit in a paper that I am ...
0
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1answer
31 views

Proving closure of unit space of a Hausdorff groupoid

For Hausdorff topological groups, the set $\{e\}$ containing only the identity is closed. This is because Hausdorff implies T1 which implies singletons are closed. For topological groupoids, defined ...
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0answers
9 views

Group of Units of a finite dimensional algebra over a local field

If we have a finite dimensional algebra $A$ over a locally compact topological field, is it true that the group of units $A^{\mbox{*}}$ is a topological group?
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0answers
39 views

Prove: If H and G/H are totally disconnected then G is also totally disconnected

Let $G$ be a topological group and $H$ a subgroup of $G$. If $H$ and $G/H$ are totally disconnected, then $G$ is also totally disconnected. With 'totally disconnected' we mean the every connected ...
1
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1answer
53 views

quotient of an amenable group

I have a question about amenable groups. The notion of amenability I am using is: The action of $G$ on $k$ ($k$ locally compact in a topological vector space) is amenable if there exists a point $x ...
0
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0answers
32 views

Choosing a canonical fundamental domain

I have a set of equations that partitions a certain space into equivalent regions. For a given point $p$ contained in region $R_1$, there are equivalence relations giving its equivalent position in ...
0
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0answers
17 views

Considering the right Haar measure on the affine group, how does the absolute value come in?

Let $ G $ be the affine group with group action defined by $ (b,a)\cdot(x,s) = (ax+b,as) $ then it is a locally compact group and as such has a Haar measure. In particular the left Haar measure is $ ...
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0answers
31 views

$\widehat{\mathbb{T}}$ can be identified with $\mathbb{Z}$

$ \mathbb{T} \stackrel{\text{def}}{=} \{ z \in \mathbb{C} : |z| = 1 \}$ $\widehat{\mathbb{T}} \stackrel{\text{def}}{=} \text{Hom}(\mathbb{T},\mathbb{T})$ To show that $\widehat{\mathbb{T}}$ can be ...
2
votes
1answer
67 views

Identifying the dual group of a locally compact abelian group with the spectrum of $ {L^{1}}(G) $.

Folland stated the following theorem in his book A Course in Abstract Harmonic Analysis on Page 88. The dual group of a locally compact abelian group can be identified with the spectrum of $ ...
2
votes
1answer
36 views

Are bounded subsets of Lie groups totally bounded

Let $ G $ be a finite dimensional real Lie group, and take a bounded ball $ B_R(e) \subset G $ in it, coming from the Riemannian metric, which itself is induced from an inner product on $ \mathfrak{g} ...
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5answers
457 views

Can $S^2$ be turned into a topological group?

I know that $S^1$ and $S^3$ can be turned into topological groups by considering complex multiplication and quaternion multiplication respectively, but I don't know how to prove or disprove that $S^2$ ...
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2answers
50 views

Is intersection of connected subgroups connected?

Let $G$ be a compact group. If $A_{\alpha}$, $\alpha \in I$ is a family of closed connected subgroups in $G$, then is it true that $\bigcap_{\alpha \in I}A_{\alpha}$ is connected?
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0answers
23 views

Summand of a subgroup in a torus

Let $\mathbb{T}^n := \mathbb{R}^n / \mathbb{Z}^n$ denote the $n-$dimensional torus. If $K$ is a closed normal subgroup of $\mathbb{T}^n$, then does there exist a subgroup $L$ of $\mathbb{T}^n$ such ...
0
votes
0answers
102 views

Measurable function implies equivalent to an exponential function.

This is a follow up to this question. In that question, I answered that an exponential function can be uniquely determined by three properties: a functional equation, a weak continuity assumption, and ...
6
votes
1answer
84 views

Profinite topology of a Group

Let $G$ be a group. Consider now the set of all (left for instance) cosets in $G$ of subgroups of finite index. This set is a base for a topology in $G$. I found somewhere that if $G$ is residually ...
1
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1answer
56 views

A paratopological group with intersection of neighborhoods of a point non-closed

Do you have an example of a (para)topological group $(G,\mathcal T)$ such that $\bigcap_{V\in \mathcal N_1} V$ is not closed? $\mathcal N _1$ is the set of all neighborbood of the identity element of ...
0
votes
0answers
67 views

Explanation of a passage in Atiyah / Macdonald

On page 105 the authors show that $\hat{\hat{G}} \cong \hat{G}$ (Proposition 10.5) and conclude that the canonical homomorphism $\phi : \hat{G} \to \hat{\hat{G}}$ is an isomorphism. How does the fact ...
0
votes
1answer
22 views

Continuity of Multiplication by Fixed Element

This is likely a simple question that I'm just missing, but nothing immediately came to mind. When dealing with topological monoids, it is necessary to prove that the group operation is continuous. ...
1
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2answers
51 views

Introduction to discrete subgroups of the euclidean group

I am looking for a general introduction to discrete subgroups of the euclidean group (= group of isometries in euclidean space). Even though I searched quite a bit, I was unable to find a good ...
2
votes
0answers
35 views

A homeomorphism of topological groups.

Let $L/K$ be a Galois field extension and $(I, \leq)$ be the directed set of all finite Galois extensions $E$ of $K$ contained in $L$ (we say $E^{\prime}\leq E$ if $E^{\prime}\subseteq E$). If ...
2
votes
2answers
50 views

Showing that every path can be well-divided?

Let $\gamma: [0,1] \rightarrow S^1$ be a path. We'll say that $\gamma$ is well-divided if there are $a_1,...a_n$ such that: $a_1=0$, $a_n=1$ $\forall_{1\leq i < n}: a_i<a_{i+1}$ ...
0
votes
0answers
33 views

Continuous maps of topological groups

Suppose $\theta: G\to K$ is a homomorphism of topological groups. We know that $G$ and $K$ each have fundamental systems of open neighbourhoods of the identity. If I want to show that $\theta$ is ...
7
votes
2answers
131 views

What topological restrictions are there for a topological space to be a group?

I'm trying to provide a group structure for some Riemannian surfaces. I heard that the following result holds: Let $X$ be a compact Riemannian surface. Then $X$ admits a group structure if, and ...
0
votes
1answer
22 views

Haar measure of point sets

Let $G$ be a locally compact group with Haar measure $\mu$ (left or right doesn't matter to me). I know that the Haar measure is positive on open sets. What can be said about the Haar measure on ...
0
votes
1answer
29 views

Elements Outside the Identity Component $SO^+(1,\,3)$ of the Lorentz Group $O(1,\,3)$

I have been answering a question on Physics Stack exchange to do with the difference between the "proper orthochronous" (i.e. identity component of) the Lorentz group $SO^+(1,\,3)$ and the Lorentz ...
2
votes
2answers
71 views

Closure of a topological group

Let $G$ be a topological group. Define $H(g)=\{g^n\}^{\infty}_{n=-\infty}$ for each $g \in G$. I need to prove that the closure of the set $H(g)$ is a commutative subgroup of $G$. But I am not sure ...
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0answers
28 views

Connected component and group action

Let $G$ be a topological group acting on a set $X$. Let $x \in X$ and consider the orbit $G.x$ endowed with the topology coming from the quotient $G/ Stab(x)$. If $G^0$ is the connected component of ...
1
vote
1answer
77 views

Topology of $GL_n(K)$

I need to show any of the following results: Consider $K=\mathbb{R}$ or $\mathbb{C}$, then, 1) The compact-open topology and the usual topology of $GL_n(K)$ are the same. 2) Taking inverses and ...
0
votes
0answers
21 views

Circular jump progrssion

in a circular pond, lotus petals are arranged along the perimeter. a frog leaps from one petal to another in such a way that starting from a petal, it skips one petal and jumps to the next one; then ...
0
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1answer
25 views

A descending filtration on a group defines a topological group.

I'm going through this Wikipedia section on Group Filtrations. Let $G$ be a group and $G_n$ a descending filtration of subgroups ($G_{n+1} \subset G_n$). I've already proved that it forms a ...
0
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2answers
34 views

Showing $U$ open in topological group $G$ $\implies$ $gU$ is open

If $G$ is a topological group, and $U$ is an open set in $G$, then do we have that $gU$ is also open in $G$? I know that since $G$ is a topological group, the mappings $\mu: G^2 \rightarrow G$ s.t. ...
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votes
2answers
34 views

Topological Rings and Homothecy

I am trying to show that in a topological ring $A$, if left homothecy $x \mapsto ax$ is continuous at $x=0$ for all $a \in A$ and multiplication $\mu$ is continuous at $(0,0)$, then multiplication is ...
3
votes
1answer
58 views

Showing that $\pi(G/H, 1) = H$ under a condition

Problem: Let $G$ be a simply connected (i.e., $\pi(G)=1$) topological group, and let $H$ be a discrete normal subgroup. Prove that $\pi(G/H,1) = H$. I know that since $H$ is a discrete subgroup ...
2
votes
0answers
56 views

a measurable function on a LCA group coincide with an mulitplicative character almost everywhere

Let $G$ be an LCA group. We say $\tilde \chi$ is a multiplicative character if it is a continuous function : $\tilde \chi : G \to S^1 $ where $ S^1 : = \{ z \in \mathbb C : |z| =1 \}$ is the unit ...
0
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0answers
77 views

Prove that the Pontryagin dual of a locally compact abelian group is also a locally compact abelian group.

Let $ G $ be a locally compact abelian (LCA) group and $ \widehat{G} $ the Pontryagin dual of $ G $, i.e., the set of all continuous homomorphisms $ G \to \mathbb{R} / \mathbb{Z} $. Clearly, $ ...
2
votes
1answer
62 views

A question on the classifying space $BG$, its universal property (?), and the stack $[\bullet/G]$

I am learning about the classifying space $BG$ of a topological group $G$. I know the definition $$BG=EG/G,$$ where $EG$ is any contractible space on which $G$ acts freely. If I am not mistaken, with ...