Locally Compact Hausdorff Abelian Groups

November 12, 2008

Locally Compact Hausdorff Abelian Groups

Posted by John Baez

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This is a followup to my post on Pontryagin duality. In the comments to that, Yves de Cornulier said:

It’s well known (and not trivial) that any locally compact abelian group $A$ has a compact subgroup $K$ such that $A/K$ is a Lie group.

This sent me back to the books. I learned some stuff I should have known a long time ago. Let me tell you about it.

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The following is some learned commentary on Theorem 25 in here:

Sidney A. Morris, Pontryagin Duality and the Structure of Locally Compact Abelian Groups, London Math. Soc. Lecture Notes 29, Cambridge U. Press, 1977.

The theorem is supposed to shed light on the structure of abelian topological groups that are locally compact and Hausdorff — the groups that allow for a massive generalization of the Fourier transform called Pontryagin duality.

Morris calls these LCA groups. Lots of people just call ‘em locally compact abelian groups, and hide “Hausdorff” in the fine print.

Anyway, here’s the theorem:

Principal Structure Theorem for LCA Groups: If $G$ is an LCA group, $G$ has an open subgroup that’s isomorphic (as a topological group) to $\mathbb{R}^n \times K$ for some finite $n$ and some compact abelian group $K$ .

The meaning of this theorem is a bit obscure at first. It’s instantly striking how $\mathbb{R}^n$ shows up out of the blue, starting from hypotheses that don’t involve the real numbers! But the theorem doesn’t deliver what you naively want, namely a classification of LCA groups.

If you’re hoping for a classification, maybe it’s because normal mathematicians only know a few easy examples of LCA groups, namely:

the real line $\mathbb{R}$
the circle $S^1$
the integers $\mathbb{Z}$
finite cyclic groups $\mathbb{Z}/n$

and products of these — possibly infinite products, but with only finitely many factors of $\mathbb{R}$ , since an infinite product of copies of $\mathbb{R}$ isn’t locally compact.

For easy examples like this, the Principal Structure Theorem is trivial, since the whole group is a product

$G = \mathbb{R}^n \times K \times D$

where $K$ is compact and $D$ is discrete. Why? Just take $\mathbb{R}^n$ to be the product of all your copies of $\mathbb{R}$ , take $D$ to be the product of all your copies of $\mathbb{Z}$ , and stuff all the rest into your $K$ . The subgroup $\mathbb{R}^n \times K$ will be open in $G$ , so the theorem holds.

There are, however, much weirder LCA groups! It’s hopeless to classify them! It’s hard to even understand some of them! Spend a few hours trying to visualize the Bohr compactification of the real line. You can do it, but here’s the last guy who succeeded:

So, before telling you about another weird example, let me point out some spinoffs of the Principal Structure Theorem.

First of all, an obvious corollary:

Theorem: If $G$ is a connected LCA group, it’s isomorphic (as a topological group) to $\mathrm{R}^n \times K$ , with $K$ compact and connected.

The point is that if $G$ is connected, any open subgroup has to be all of $G$ .

This has a further corollary that Todd Trimble pointed out to me:

Theorem: If $G$ is an LCA group, there is a short exact sequence of groups $0 \to G_0 \to G \to \pi_0(G) \to 0$ where the connected component of the identity of $G$ , denoted $G_0$ , is isomorphic (as a topological group) to $\mathbb{R}^n \times K$ , where $K$ is compact and connected.

Now this is very nice. However, if you aren’t paying careful attention, you may be lulled by the easy examples into believing this:

False Theorem: If $G$ is an LCA group, it’s isomorphic (as a topological group) to $\mathbb{R}^n \times K \times \pi_0(G)$ with $K$ compact and connected.

This is wrong! In fact, it’s contradicted by this result of Morris:

Theorem: Not every LCA group $G$ is isomorphic (as a topological group) to $\mathbb{R}^n \times K \times D$ with $K$ compact and $D$ discrete.

Let’s see how he shows this. He exhibits a counterexample that provides a tiny window into the world of weird LCA groups.

Let $G$ be a countable product of copies of $\mathbb{Z}/4$ . With its product topology, $G$ is compact. But Morris will give it a sneakier topology!

$G$ has a subgroup $H$ consisting of a countable product of copies of $\mathbb{Z}/2$ , one copy sitting inside each copy of $\mathbb{Z}/4$ .

Morris puts the product topology on $H$ , making it a compact totally disconnected topological group. Totally disconnected means that each point is its own connected component. There are lots of spaces that are totally disconnected but not discrete: the rational numbers are one, and this $H$ is another. But this $H$ is also compact, thanks to Tychonoff’s Theorem.

Next he puts a sneaky topology on $G$ . He chooses a base of open neighborhoods of the identity in $G$ that consists of all open sets in $H$ containing the identity.

The sneaky topology is a lot finer than the product topology! The sequence

$(1,0,0,0,...)$ $(0,1,0,0,...)$ $(0,0,1,0,...)$

never gets into $H$ , so it doesn’t converge to $0 \in G$ in the sneaky topology. It would in the product topology.

He claims that now $G$ is a totally disconnected locally compact abelian group with $H$ as an open subgroup. I guess all those statements are obvious if you think about each one for a minute!

Now, the Principal Structure Theorem says $G$ has an open subgroup isomorphic (as a topological group) to $\mathbb{R}^n \times K$ with $K$ compact.

Of course $n = 0$ , so let’s forget about the $\mathbb{R}^n$ stuff.

So the theorem says: $G$ has an open subgroup $K$ that’s compact.

I suppose $H$ itself is such an open subgroup! The identity is not such a subgroup

Then he says: suppose $G$ were a compact group $K$ times a discrete group $D$ :

$G = K \times D$

Then we get a contradiction. Since $G$ is not compact and $K$ is, $D$ must be infinite. But this is impossible because:

1) every infinite subgroup of $G$ has infinitely many elements in $H$

and

2) every discrete subgroup of $H$ is finite.

Think about it.

But what’s the point? We’ll he’s basically saying we’ve got an LCA group $G$ that’s not of the form $\mathbb{R}^n \times K \times D$ with $K$ compact and $D$ discrete.

So, it sure as heck ain’t of the form $\mathbb{R}^n \times K \times \pi_0(G)$ with $K$ compact and connected!

I haven’t reached the point of talking about Yves de Cornulier’s claim, and there’s a lot more fun stuff to say. For example, despite the impossibility of classifying all LCA groups, we can classify them if we throw on some other conditions.

But, this is about as much as anyone could be expected to read in one sitting! The takeaway point is: if you know and love the classification of finite abelian groups, and you know and love some topology, or maybe Fourier transforms, you should get to know Pontryagin duality, and learn a bit about LCA groups. Some of them are very familiar, and others are quite scary — but they sit at a nice intermediate spot between ‘too simple to be interesting’ and ‘too complicated to comprehend’. E. H Gombrich said it well:

Aesthetic delight lies somewhere between boredom and confusion.

Posted at November 12, 2008 2:46 AM UTC

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37 Comments & 1 Trackback

Re: Locally Compact Hausdorff Abelian Groups

On the minor point of the “Hausdorff” question. On Wikipedia (or in Hewitt and Ross, if you can find a copy of this old book) you’ll find that a topological group which is T_0 is already Hasudorff. Now, a T_0 space is one with a very weak separation property: given any two distinct points, there is an open set containing precisely one of them. So a topological group which isn’t Hausdorff is already very badly behaved. I think this is why the Hausdorff condition gets pushed aside.

That, and most people these days take “Hausdorff” as being necessary for a space to be locally compact! But this is one of those points (like: “Is 0 a natural number?”) which we could argue about forever, to no particularly conclusion…

Have you looked at the book “Locally compact groups” by Markus Stroppel? I didn’t like it very much, but it’s a new book, concentrating on the Abelian case, and so might be of interest. The MathSciNet reference is MR2226087.

Posted by: Doormat on November 12, 2008 10:58 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

When I become ruler of the universe, I’ll make up new words that mean ‘compact Hausdorff’ and ‘locally compact Hausdorff’, and make everyone use those — these concepts are important enough to deserve unambiguous terms.

In the meantime I just wish I could instantly tell what any given author means when they say ‘locally compact’: does it include Hausdorff, or not? Someday math may be written in a way where you can click on any term and find the author’s definition of that term; that would make me very happy.

Doormat wrote:

Have you looked at the book “Locally compact groups” by Markus Stroppel? I didn’t like it very much, but it’s a new book, concentrating on the Abelian case, and so might be of interest.

Thanks — no, I haven’t seen that. Why don’t you like it? Does it have a bunch of new results?

I find that Armacost’s book is a good compendium of results on LCA groups that goes way beyond the basic theory: the Principal Structure Theorem and Pontryagin duality appear right at the beginning, without proof, and it goes on from there…

For more introductory stuff, Sidney Morris’ book mentioned above is nice.

Posted by: John Baez on November 12, 2008 5:34 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

John said, “When I become ruler of the universe, I’ll make up new words that mean ‘compact Hausdorff’ and ‘locally compact Hausdorff’, and make everyone use those — these concepts are important enough to deserve unambiguous terms.”

I couldn’t agree more. I think that one of the reasons that some writers omit mentioning that they are dealing with Hausdorff spaces is the avoidance of tedious repitition. It’s quite monotonous to write or read “let X be a compact Hausdorff A, B, C” over and over again. So, a single short name for compact Hausdorff would relieve that situation somewhat. One would hope for a nice elegant name for these two properties, avoiding ugly ones like clopen. Of course it helps to state that all spaces will be assumed to be Hausdorff somewhere in the document or section or chapter, but then the writer has to worry that the reader notices or remembers that.

The other reasons, of course, are just plain laziness or lack of appreciation of the fact that the Hausdorff property confers unique convergence on a space, and things get really messy without that.

Posted by: Richard on November 13, 2008 3:21 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

About the Stroppel book: it’s been a while since I read it. But as I recall, I thought it unnecessarily introduced a lot of notation (which made it hard to dip into) and it seemed to rather concentrate on the Abelian case, whereas I was, at the time, more interested in the compact case. However, this fact made me think that maybe you would find it interesting. That, and it’s new, so there is more chance of libraries having it…

I find Hewitt and Ross to be the bible for topological groups, but like any bible, it can be close to impossible to read…

Posted by: Doormat on November 13, 2008 10:16 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

I’ve just been back to your original post about LCA groups, and I see that you found the Hausdorff stuff for yourself. Sorry: it must have seemed like I was stating the obvious!!

I don’t wish to oversell this Stroppel book: it seems like you’ve found a load of books yourself, so probably this new one has nothing to add.

So, a question about this Hausdorff business. A space failing to be T_0 is a space where there are two distinct points which cannot be told apart by the topology (and hence cannot be “seen as different” by any continuous function) as any open set which contains one contains the other. Do topological groups failing this condition really “arise in nature”? Maybe in algebraic geometry (of which I know embarrassingly little)? From my naive point of view, it would seem that topology, and hence continuous functions, were the “wrong tool” for such objects…

Posted by: Doormat on November 13, 2008 1:48 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

Non-Hausdorff topological spaces certainly arise in nature. E.g., on p. 55 of Connes’ book he talks about the spaces of leaves of a foliation with the quotient topology failing to be Hausdorff. Hence the trick of studying the associated convolution algebra of functions on the associated groupoid (p. 105).

And the space of leaves on that foliated torus (p. 55) is a group.

Posted by: David Corfield on November 13, 2008 2:43 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

It’s probably a reasonable statement that the most important (and mundane) non-Hausdorff spaces in mathematics are algebraic varieties with the Zariski topology. One gets around the pathology associated with this by making the topology on products of varieties finer than the product topology, so that, for example, the diagonal is closed. There’s nothing artificial about this process, because the result is the product in the category of varieties.

Posted by: Minhyong Kim on November 13, 2008 3:53 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

I forgot to add that continuous functions indeed are not the right tool in algebraic geometry. But the right functions (algebraic ones) are still continuous. Also, very importantly, the Zariski topology is good enough for developing a good sheaf theory, at least for suitable sheaves (the ones that are like algebraic functions).

Posted by: Minhyong Kim on November 13, 2008 3:58 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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Do topological groups failing this condition [T₀] really “arise in nature”?

Sure, but (as Gabor describes in point 5 of his comment) you usually mod out by a subgroup to make it T₀ (and hence T_3½).

Famous example: The topological vector space (hence topological abelian group) of square-integrable almost-everywhere defined functions (with given appropriate source and target spaces). Two functions that are equal almost everywhere are topologically indistinguishable, yet they need not be equal. So to form the usual space ℒ², you mod out by the almost-everywhere vanishing functions to get a T₀ (hence also T_3½) space.

This trick is really purely topological, and is often done whenever one comes across a space that might not be T₀; see this Wikipedia article.

Posted by: Toby Bartels on November 14, 2008 7:13 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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I looked into the EL Naschie thread and got a bit depressed, so it’s good have some mathematics back!

I’m not sure what the general goals are in the book you cite, but I hope it mentions more natural examples of locally compact abelian groups like the $p$ -adic numbers $\mathbb{Q}_p$ . Perhaps the point is that if you look at this group from a ‘Euclidean perspective’ it is terribly strange. (Topology related to a Cantor set, I guess.)

Posted by: Minhyong Kim on November 12, 2008 12:28 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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Hi, Minhyong! The book by Morris cited above is very elementary (that’s its charm) and gives rather few examples; I don’t see it mentioning LCA groups related to the $p$ -adics.

This book goes much further:

David A. Armacost, The Structure of Locally Compact Abelian Groups, Dekker, New York, 1981.

and it talks quite a bit about LCA groups related to the $p$ -adics. You’re right, these are more interesting than the weird example I gave above. But I haven’t thought about them much, e.g. learned about their Pontryagin duals.

Posted by: John Baez on November 13, 2008 10:08 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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I make embarrassingly many mistakes on these elementary questions, but I guess the Pontryagin dual of $\mathbb{Q}_p$ is itself, using the same pairing as for the reals. That is, we check that

$\exp(2\pi i x)$

makes sense for a $p$ -adic number $x$ , and is a $p$ -power root of 1. Then we can construct a perfect pairing

$\mathbb{Q}_p\times \mathbb{Q}_p\rightarrow S^1$

that sends $(x,y)$ to $\exp(2\pi i xy).$

Execise: What is the Pontryagin dual of the $p$ -adic integers $\mathbb{Z}_p$ ?

Posted by: Minhyong Kim on November 13, 2008 11:11 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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< imitates Horshack from Welcome Back, Kotter > Ooh, ooh – I know!

Hint # 1: what do you know about the $p$ -adic integers as a topological space?

Hint # 2: thinking of the categorical construction of the $p$ -adics, and in view of the first hint, think of a group whose dual would give the $p$ -adics.

Posted by: Todd Trimble on November 14, 2008 2:22 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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Yes! Implicit in this hint is the interesting fact that any continuous homomorphism from $Z_p$ to $S^1$ factors through a finite quotient $Z/p^n$ . This is exactly related to the absence of non-zero compact subgroups of $R$ .

A nice generalization is the standard fact that a continuous complex representation of a pro-finite group (e.g., the automorphism group of the algebraic numbers) has to factor through a finite quotient.

Posted by: Minhyong Kim on November 20, 2008 4:50 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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It’s instantly striking how $\mathbb{R}^n$ shows up out of the blue, starting from hypotheses that don’t involve the real numbers!

Any idea why it does? Could it be linked to a characterisation of $\mathbb{R}$ ? Maybe as complete Archimedean ordered field or final coalgebra of the functor, ordinal product with $\omega$ .

Something I mean to dig into soon is category theoretic characterisations of the reals.

Posted by: David Corfield on November 12, 2008 3:34 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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This isn’t a real answer to your question, but part of the reason for my comment above was the remark that you quote. That is, the case of $n=0$ is when $R$ doesn’t show up at all. So there are two possible shallow answers to the question of why $R$ shows up:

1. It doesn’t, necessarily;

2. You have to list it as a possible separate factor because it’s a group that doesn’t have a compact open subgroup.

Posted by: Minhyong Kim on November 12, 2008 4:20 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

So can anything be said in general about locally compact abelian groups that don’t have a compact open subgroup?

Posted by: David Corfield on November 12, 2008 4:43 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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As I understand it, the theorem is saying that any such thing that’s ‘irreducible’ is $R$ .

Posted by: Minhyong Kim on November 12, 2008 6:12 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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So we get a new definition of $\mathbb{R}$ !

Posted by: Tim Silverman on November 12, 2008 9:02 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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David wrote:

So can anything be said in general about locally compact abelian groups that don’t have a compact open subgroup?

I think I’ll follow Minhyong’s advice and change your question to one with a better answer.

Let’s define a Corfield group to be an LCA group without any compact open subgroups. And let’s say a Corfield group is irreducible if it doesn’t have a proper subgroup that’s itself Corfield.

Note: we give the subgroup its induced topology when asking if it’s Corfield.

Note: $\mathbb{R}$ is Corfield, since any open subgroup would contain a neighborhood of $0$ , forcing the subgroup to be all of $\mathbb{R}$ .

Now suppose $G$ is an irreducible Corfield group. By the structure theorem $G$ has an open subgroup of the form $K \times \mathbb{R}^n$ with $K$ compact. We must have $n = 0$ or $n = 1$ , else $G$ would have $\mathbb{R}$ as a proper Corfield subgroup.

If $n = 0$ , $G$ has $K$ as a compact open subgroup. This contradicts the assumption that $G$ is Corfield.

So we must have $n = 1$ . This means $G$ has $\mathbb{R}$ as a subgroup. $\mathbb{R}$ can’t be a proper subgroup since $G$ is irreducible Corfield. So $G = \mathbb{R}$ .

Moral: Up to isomorphism, $\mathbb{R}$ is the unique irreducible Corfield group.

Posted by: John Baez on November 13, 2008 4:29 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

Does this characterisation of the reals relate to any other, especially category theoretic ones? What’s doing most of the work in forcing the answer to be the reals, or is it a combination of the algebraic and the topological properties?

Is there any way to phrase some of these properties category theoretically: compact, noncompact, locally compact, no compact open subspace?

Posted by: David Corfield on November 13, 2008 11:18 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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I don’t have terribly insightful answers to any of these questions. I think it’s impossible to say either algebra or topology is doing ‘most of the work’ in the Principal Structure Theorem for LCA groups.

Here’s one moral though: a group acts on itself by left translation, making any point ‘look just like any other’ once we forget which point is the identity. In other words, it’s a very homogeneous thing. For a topological group, any slightly nice topological property gets massively amplified by this homogeneity.

Even better, any neighborhood $U$ of the identity has a ‘square root’ — an open neighborhood $V$ such that the product of two elements in $V$ lies in $U$ . We can also choose neighborhoods $U$ that are ‘symmetrical’ in the sense that the inverse of an element in $U$ again lies in $U$ . Todd illustrated the power of these ideas by showing any T₁ space (where for any two distinct points there is an open set containing the first but not the second) is T₂, also known as Hausdorff (any two disinct points lie in disjoint neighborhoods).

This is just one link in a long chain of results where ever-so-slightly-nice topological groups are forced to be vastly nicer than you’d at first suspect!

This line of thought was epitomized by Hilbert’s problem: is a topological group that’s a topological manifold actually a Lie group? So: are the continuous group operations actually smooth?

The answer is yes… but in their work on this problem, people eventually found much stronger results. A famous one is due to Gleason, Montgomery and Zippin: any locally compact Hausdorff group with no small subgroups is a Lie group!

Posted by: John Baez on November 13, 2008 4:22 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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In case anyone is still interested, I guess I’ve figured out why the impression that $R^n$ ‘suddenly shows up’ didn’t seem quite right to me. The fact that $R^n$ has to *occur* in any such classification theorem is evident, since the definition of a locally compact topological group starts out with examples like $R^n$ . That is, its occurrence is ‘built-in.’ So the actual point of interest is that it’s quite different from the other examples, and hence, has to be put in as a separate factor in the general form

$R^n\times K\times D.$

This creates the impression that it’s somehow ‘appeared.’

Consider for comparison the structure theorem that says any finitely-generated abelian group has the form

$Z^n \times$ (a finite abelian group in some standard form)

We shouldn’t feel like $Z^n$ came up magically. (I hear a psychologist chiding me for telling others how they should feel.)

Posted by: Minhyong Kim on November 20, 2008 4:27 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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Minhyong wrote:

(I hear a psychologist chiding me for telling others how they should feel.)

Heh. I don’t think it’s so terrible to allow ourselves a little private moment of excitement at how $\mathbb{Z}^n$ stands out in the structure theorem that says any finitely generated abelian groups looks like

$\mathbb{Z}^n \times finite abelian group.$

But sure: to say it appeared magically would be ridiculous. The underlying dichotomy is that a torsion-free finitely generated abelian group is $\mathbb{Z}^n$ , while a pure torsion finitely generated abelian group is finite. Both these facts are pretty darn easy to see.

It’s a bit more magical the way $\mathbb{R}^n$ stands out in the classification of locally compact Hausdorff abelian groups. If I’m not mistaken, the reason is that the $\mathbb{R}^n$ ’s are the only groups of this sort that are connected and lack compact subgroups. And this is less easy to see.

To make this more of a conversation about math and less of a conversation about ‘feelings’:

Is there a structure theorem about locally compact Hausdorff abelian groups that puts the groups $\mathbb{Q}_p$ on a more equal footing with $\mathbb{R}$ ?

Maybe something sort of ‘adelic’, that treats $\mathbb{R}$ as the curiously defective $\mathbb{Q}_p$ corresponding to the real prime?

I think I saw someone talk about the ‘ $p$ -adic rank’ of a locally compact Hausdorff abelian group. Could this notion help?

Posted by: John Baez on November 20, 2008 4:57 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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‘Is there a structure theorem about locally compact Hausdorff abelian groups that puts the groups $Q_p$ on a more equal footing with $ℝ$ ?’

My understanding of this question in the present context is that we are asking about a structure theorem that says, for example, that any locally compact Hausdorff abelian group is of the form

$Q_3\times G\times D,$

where $D$ is discrete and $G$ has property $P$ . Of course, $P$ should be a property that allows us to put all the other $Q_p$ ’s as well as $R$ into that factor.

This is rather interesting, except it’s hard to say exactly what class of properties we are allowing for $P$ . If we allow, for example,

$P:$ $G$ contains no subgroup isomorphic to $Q_3$

it wouldn’t be so interesting. On the other hand, if we allow only *topological* properties, we probably can’t state such a theorem. (I believe all the $Q_p$ ’s are homeomorphic.)

So it’s something of a challenge to come up with an interesting theorem of this sort.

Posted by: Minhyong Kim on November 20, 2008 11:03 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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What is it about its being a ‘curiously defective $\mathbb{Q}_p$ ’ that makes the reals be so different, e.g., not being totally disconnected; having an Archimedean metric; having an algebraic completion of finite degree, which is unique and metrically complete; being ordered?

Are all these odd properties related?

Posted by: David Corfield on November 21, 2008 10:15 AM | Permalink | Reply to this

Re: Separation axioms in topological groups

I am sure I am going to state trivial facts, but in light of the discussion on the T_0 property, I feel that these should be mentioned:

1. While T_0, T_1, T_2, T_3, and T_{3.5} are distinct properties for topological spaces, this is not the case for topological groups.

2. Every topological group is regular even if it is not T_0.

3. Every T_0 topological group is T_{3.5} (i.e., Tychonoff).

4. If G is a non-T_0 topological group, then N=cl_G {0} is the smallest closed subgroup of G, and G/N is the maximal Hausdorff quotient of G.

5. G and G/N have the same continuous characters, because T is Hausdorff (and the construction described in 4 is a reflection).

The conclusion that I draw from these is that, as far as duality is concerned, non-T_0 groups are not interesting.

Posted by: Gabor Lukacs on November 14, 2008 4:25 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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Is $U(1)$ the dualising object for Pontryagin duality because it’s ambimorphic (neé schizophrenic)?

Posted by: David Corfield on November 14, 2008 8:49 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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In view of the nature of this forum, perhaps it’s good to remind ourselves that Pontryagin duality deals with mere abelian groups. My feeling is that the correct generalization to non-abelian groups is far from understood. One way is to say that the dual is some category of representations. But to make a far-fetched demand, it would be nice to have a dualizing object that relates well to the entire category of non-abelian groups. For this, perhaps one needs to have a serious understanding of the role of $S^1$ .

Posted by: Minhyong Kim on November 14, 2008 10:40 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

What about the approach using Kac algebras (or, better still, Locally Compact Quantum Groups). Here’s a nice overview in PNAS by Van Daele.

Briefly: you instead study a suitable _algebra_ of functions on G instead of G itself. Which is cheating, in one sense, but does seem pretty elegant to me.

Posted by: Doormat on November 14, 2008 11:11 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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This is an interesting point that I hadn’t contemplated. Is the dual some version of the dual Hopf algebra? I presume not, since that would be like taking

$Hom(\mathbb{R},\mathbb{R})$

rather than

$Hom(\mathbb{R}, S^1)$

(even though they end up being the same).

Posted by: Minhyong Kim on November 15, 2008 9:38 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

Is the dual some version of the dual Hopf algebra? I presume not, since that would be like taking
Hom(ℝ,ℝ)
rather than
Hom(ℝ,S 1)

This is way outside my area of competence – apologies/warning in advance – but I believe that the LCQG approach does indeed take a suitable von-Neumann algebra flavoured version of the dual-Hopf algebra-like-object. In the case where your group is finite, I think the dual of the cocommutative Hopf algebra kG is the commutative Hopf algebra G^k? and if memory serves right, the LCQG approach generalizes this.

(There are some notes on the arXiv by van Daele which attempt to give extra motivation and exposition of the compact quantum group case, which in this setting would be a discrete abelian group.)

Re your comment: Hom(_, S¹) seems to arise because you want bounded Homs of some sort in the category of suitably-topologized Hopf algebras. But this is a very hazy thought on my part and might be completely mistaken…

Posted by: yemon choi on November 15, 2008 6:13 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

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correction: the dual of kG is not what I said (or at least, it’s not clear to me that what I wrote makes sense).

Posted by: yemon choi on November 19, 2008 12:08 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

$MathML-enabled post (click for more details).$

Peter Johnstone in Stone Spaces (p. 262) speaks of $T = \mathbb{R}/\mathbb{Z}$ as an abelian group object in the category of compact Hausdorff abelian groups.

Then we need to know lots of complicated things such as that $T$ is injective in AbGp and that it contains an isomorphic copy of every cyclic group so has enough $T$ -valued characters. And corresponding properties for the category of compact topological abelian groups, which will involve integration on compact groups as part of the Peter-Weyl theorem.

On the other hand, $2$ as a dualising object seems to live a charmed life: set, complete Boolean algebra, compact Hausdorff boolean algebra, topological space, frame, complete semilattice, boolean algebra, partially ordered topological space, coherent space, lattice, poset, Stone semilattice, and more. Why is life so easy for $2$ ?

Up a level will $Sets$ ever get used so ambimorphically?

Posted by: David Corfield on November 14, 2008 11:18 AM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

Todd gave me a good pointer about my last question back here.

Posted by: David Corfield on November 15, 2008 10:41 PM | Permalink | Reply to this

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Tracked: November 20, 2008 9:55 AM

Re: Locally Compact Hausdorff Abelian Groups

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I’m getting the feeling that there should be some structure theorem for LCA groups that’s more detailed than the Principal Structure Theorem quoted above.

I know a very nice theorem for LCA groups that are ‘compactly generated’ — generated by a compact subset. Namely, they’re all isomorphic to

$\mathbb{R}^n \times \mathbb{Z}^m \times K$

with $K$ compact.

And I know what people say: compact abelian groups are hopelessly unclassifiable, since their Pontryagin duals are discrete abelian groups — or in other words, abelian groups, plain and simple — and these are a hopeless morass.

But still, might there not be some way to chop up $K$ into parts with different properties, that sheds a bit more light on its structure?

Also: what more can we say about the structure of locally compact Hausdorff abelian group that’s not necessarily compactly generated? Surely the Principal Structure Theorem isn’t the last word on this subject.

Posted by: John Baez on November 20, 2008 5:10 PM | Permalink | Reply to this

Re: Locally Compact Hausdorff Abelian Groups

$MathML-enabled post (click for more details).$

Faisal at Mathoverflow finally settled the question that triggered this post! Yves de Cornulier had written:

It’s well known (and not trivial) that any locally compact abelian group $A$ has a compact subgroup $K$ such that $A/K$ is a Lie group.

I’d never found a proof, which was getting embarrassing, since I have a book almost finished which needs this result.

But as Faisal kindly noted, Corollary 7.54 in The Structure of Compact Groups by Hoffman and Morris does the job:

If $A$ is an LCA group, then each neighborhood of the identity contains a compact subgroup $K$ such that $A/K \cong \mathbb{R}^m \times \mathbb{T}^n \times D$ where $D$ is a discrete abelian group.

So, $A/K$ is a Lie group by my definition. ( $A/K$ may have any number of connected components, even uncountably many, and they may be 0-dimensional.)

Posted by: John Baez on February 8, 2011 5:51 AM | Permalink | Reply to this

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November 12, 2008