# Topological groups, why need them?

I'm reading through Munkres and Armstrong's books on topology. However, I find topological groups to be really complicated objects! I feel they are twice as hard to deal with then just groups and topological spaces, but why do we need it? Can anybody help me?

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Topological groups are used to study continuous symmetries, and are used very frequently in modern theoretical physics. –  Sanath Devalapurkar Apr 16 at 2:16
@FlybyNight I just thought that it'd be too small an answer. However, I will post it as an answer now. –  Sanath Devalapurkar Apr 16 at 2:39

The reason that $<$concept$>$ is important is that there are enough examples of $<$concept$>$ to justify making the definition. So I will answer this by giving some examples. The first example that we come across is the real numbers, $(\mathbb{R}, +)$ (by extension all the $\mathbb{R}^n$). We see that the map $sub:\mathbb{R}^2\to \mathbb{R}$ which is defined by $(x,y)\mapsto x-y$ is continuous. Thus $\mathbb{R}$ is a topological group. One example is not enough to justify a definition, so here are a few more:

• All of the matrix groups over $\mathbb{R}$ and $\mathbb{C}$ (and also all of the Lie groups).

• The circle $S^1$, and all the tori, $(S^1)^n$ (these are special cases of Lie groups, but they are important to mention on their own)

• Let $X$ be any (reasonable, see the comment by Mike for a notion of reasonable) space. Then the set of homeomorphisms of the space, which we will denote $\operatorname{Homeo}(X)$ may be topologized (with the compact open topology) and thus is a topological group. (This one is a little more involved than the first two.)

• Any topological vector space, including Banach spaces and Hilbert spaces. These are important because they come up in functional analysis all the time.

Now, you also say that you find the notion of topological groups difficult. Well, let us suppose that we understand spaces and groups separately. Then all these topological groups are spaces that have a group structure such that all of the group structure is continuous. We may slickly define this as a space $X$ such that that we have a group multiplication on $X$, which we will call $*$, such that the map $$\phi:X^2\to X$$ defined by $$(x,y)\mapsto x*y^{-1}$$ is continuous. To help you understand this definition, I would advise you to verify for yourself that the examples above are indeed topological groups (with the possible exception of the third, which I would defer until you know about the compact open topology).

The fact that the group structure is continuous gives our space a property called homogeneity. Roughly, this means that every point "looks" like very other point. To be more precise, every point looks like the identity element. What this really means if we are not being vague is that if we have a small neighborhood of the identity element $U_e$, then, using the continuity of the group operation, we may find a homoeomorphic neighborhood of $g$ for any element $g$, which we will call $U_g$. We define $U_g$ to be $$\{h\in X:g^{-1}h\in U_e\}.$$ A nice exercise is to show the map, $$\theta:U_g\to U_e$$ given by $$g_2\mapsto g*g_2$$ is indeed a homeomorphism.

To see this homogeneity in $\mathbb{R}^2$, let us suppose that we have a small neighborhood of the origin, say $$\{(x,y)\in\mathbb{R}^2: x^2+y^2<\epsilon\}$$ for some $\epsilon$. We also have a small neighborhood of any another point. For example, the neighborhood of $(4,5)$ is given by $$\{(x,y)\in\mathbb{R}^2: (x-4)^2+(y-5)^2<\epsilon\}.$$ Moreover, the map, $$(x,y)\mapsto (x+4, y+5)$$ is the continuous map that takes the first neighborhood to the second.

In conclusion, here are some slides of a talk that might give additional insight, or at least some other new ideas.

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By the way, there are other examples that are important such as the $p$-adics and the Cantor set (with a certain choice of multiplication). –  Baby Dragon Apr 16 at 2:35
I feel like this answer leaves open OP's core question: what are they good for - or, alternately, why do we care that a group is topological? Unfortunately, showing a bunch of examples doesn't really give a lot of motivation as to how this extra structure helps. –  Steven Stadnicki Apr 16 at 2:35
It was helpful, and the fourth example was something I never knew of :) I'll go verify the 2nd one! –  user48078 Apr 16 at 2:52
I think one needs some assumption on $X$ in order to make $\mathrm{Homeo} (X)$ a topological group – the compact–open topology doesn't always have all the expected properties! –  Zhen Lin Apr 16 at 7:40
@ZhenLin This MathOverflow question confirms your thought. (It is a topological group for $X$ compact, though.) I personally think $\pi_0(\text{Homeo}(X))$ is the more interesting group related to that, but it's only topological in the most boring way :) –  Mike Miller Apr 16 at 18:11

To a topologist, topological groups are interesting in their own right. The group structure actually gives us interesting topological structure, too! One interesting fact is that the fundamental group (an important topological invariant) of a topological group is Abelian, a fact that spectacularly fails to be true in general - any group can be the fundamental group of some space (proof).

To an analyst or a number theorist, certain kinds of topological groups have incredibly useful properties. Locally compact groups are known to have a Haar measure, a measure that (in some sense) respects the group operation (proof; I can't advise reading it, honestly - it's not one of my favorite proofs). A group having a Haar measure means we can import topics from measure theory and essentially do analysis on this group (in particular, Fourier analysis). One important application of this is in algebraic number theory, where the Haar measure lets us do a lot of things we wouldn't be able to do otherwise.

I've heard that the representation theory of topological groups is particularly nice, but I don't have a reference for this that I've read, so take it as you will; but this fact might make them interesting to an algebraist. In particular, one can recover quite a lot of the representation theory of finite groups by working instead with compact groups; there's a powerful proof technique where one 'averages' over the group, and the Haar measure lets one do this for compact groups, too.

The combination of the group structure and the topological structure is very powerful, and makes these objects helpful to mathematicians of all stripes whenever they appear!

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So complete! I've heard of Haar measures, but didn't think it would be related to topological spaces! Thanks for the surprisingly thorough answer :) –  user48078 Apr 16 at 9:51
@user48078 Glad to help - and that's only a little bit of how interesting they are! If you study much algebraic topology, you'll see they're some of the nicest spaces we can have. –  Mike Miller Apr 16 at 17:12
The Haar measure is nice but good god the proof is brutal. The ideas aren't hard but... extremely technical. The representation theory of locally compact abelian groups is quite nice and you can do some stuff in general but there's a lack of machinery for all LCGs. The dual group is quite a fascinating thing and is a bit surprising. –  Cameron Williams Aug 11 at 0:14
@CameronWilliams I quite like the proof using Kakutani's fixed point theorem of Haar probability measures on CGs - shame it doesn't extend at all to the general case :) Do you have a reference for your comment about LC abelian groups? I'm afraid essentially all the representation theory I know is from Serre's book on finite groups. –  Mike Miller Aug 11 at 0:17
Surprisingly (if you know a some functional analysis), Rudin's text Fourier Analysis on Groups is very readable, I think. He doesn't try to make things as abstract as possible which is quite refreshing. Folland's text Abstract Harmonic Analysis is also good, but Folland does things in the most abstract setting possible. However it is the most self-contained text you'll get I feel. –  Cameron Williams Aug 11 at 0:19

Converted from comment to answer: Topological groups are used to study continuous symmetries, and are used very frequently in modern theoretical physics. Local symmetries form the basis for gauge theories - QED is an abelian gauge theory with the symmetry group $U(1)$, quantum chromodynamics, is a gauge theory with the action of the $SU(3)$ group, and the main attraction: the Standard Model is a non-abelian gauge theory with the symmetry group $U(1)×SU(2)×SU(3)$. As you can see, topological groups are used to study continuous symmetries, and are used very frequently in modern theoretical physics. Indeed, there are many more examples, but an enlarged post be would be more appropriate for Physics. S.E.

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WOW This is an answer I never would have heard elsewhere! That is exciting :) –  user48078 Apr 16 at 2:54
@user48078 I find the usage of topological groups in physics fascinating, too. Are you learning topological groups for a mathematical purpose or for a physics purpose? I can provide some good references for either/both. –  Sanath Devalapurkar Apr 16 at 2:55
I was studying for math purpose, but I would appreciate both, since I am interested in physics too :) –  user48078 Apr 16 at 3:00
@user48078 I'll see if I can compile a list - I'll get back to you, probably tomorrow. Until then, check out the book at this question –  Sanath Devalapurkar Apr 16 at 3:02
Thank you so much for your help! –  user48078 Apr 16 at 3:07

Topological groups also arise out of purely algebraic situations. Here are just two of the most common examples.

In number theory one considers the rings of $p$-adic numbers $\mathbb Z_p$ for primes $p$. I won't get into their construction except to note that their topology is built out of purely algebraic considerations, and that $\mathbb Z_p$ is in fact a topological ring, not just a topological group, with field of fractions $\mathbb Q_p$ (a topological field). The topology of these rings/fields greatly simplifies the theory of their algebraic extensions, and we can understand many things about $\mathbb Z$ and $\mathbb Q$ once we understand $\mathbb Z_p$ and $\mathbb Q_p$ for all primes $p$. (We also need to consider $\mathbb R$, which is also a topological field.)

Here's another example. The Galois correspondence between subgroups and intermediate field extensions breaks down when one looks at infinite extensions--there are just too many subgroups to hope for a bijective correspondence. But Galois groups can be expressed as inverse limits of finite groups, which carry the Krull topology. And it turns out that the closed subgroups in this topology correspond to intermediate extensions, with open subgroups corresponding to the finite subextensions (yes, open subgroups are closed).

In the above examples, the topological spaces are a bit weird (they look like Cantor sets--in fact $\mathbb Z_2$ is homemorphic to Cantor's middle thirds set), but their topological structure is exactly what's needed to make sure these groups measure what they are supposed to.

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Number theorists, please don't pounce because I appear to imply that the discipline is "purely algebraic!" All I mean here is that all you need is basic theory of primes in algebraic extensions before local fields enter the picture. –  Brett Frankel Apr 16 at 4:59
So they arise in number theory? I've never went through p-adics or related topics yet, but it was very illuminating :) I think I'll read this again some days later.. :) –  user48078 Apr 16 at 5:35