# Talk:Commutator subgroup

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## Image under homomorphism

I changed f(G') being a subset of H' by subgroup (since G' is a subgroup, not just the set of generators (it's the generated ("spanned") subgroup)) and thus f(G') is a subgroup of H. But f(G') is contained in the subgroup H', and thus taking the intersection H' \cap f(G') gives the subgroup f(G') drini 05:31, 31 May 2005 (UTC)

## Title

Why is this "derived group" and not "derived subgroup"? Dysprosia 09:10, 13 February 2006 (UTC)

I think it would be better to call the article derived subgroup (or commutator subgroup). I also question the notation ${\displaystyle G^{1}}$. I don't think I've ever seen this notation before - usually ${\displaystyle G'}$ or ${\displaystyle [G,G]}$ is used, or sometimes (in the context of the derived series) ${\displaystyle G^{(1)}}$. --Zundark 12:54, 14 February 2006 (UTC)
I agree with you (derived subgroup should be fine, I think that is more common, but I haven't done a Google-check). Dysprosia 07:10, 15 February 2006 (UTC)
I've just tried Google (Web, Groups and Books), and also Zentralblatt. All four of these searches indicate that "commutator subgroup" is more significantly more common than "derived subgroup". (Some of these seaches get more hits for "derived group" than for "derived subgroup", but this could be due to the non-mathematical usages. I don't think there is any non-mathematical use of the expression "commutator subgroup".) So I suggest we move the article to commutator subgroup. --Zundark 08:49, 15 February 2006 (UTC)
Sounds fine, then. Go for it. Dysprosia 11:59, 15 February 2006 (UTC)
OK, I've moved the article to commutator subgroup. --Zundark 12:47, 15 February 2006 (UTC)

## Notation

As pointed out above, the notation ${\displaystyle G^{1}}$ is unusual. If there are no objections, I intend to change it to ${\displaystyle [G,G]}$ (because the alternative ${\displaystyle G'}$ is too easily misread as ${\displaystyle G}$ in some fonts). --Zundark 12:49, 15 February 2006 (UTC)

The notation ${\displaystyle G^{(n)}}$ is nice in some respects because the condition for a group to be solvable is that there exists some n such that ${\displaystyle G^{(n)}=\{e\}}$, echoing ideas of differentiation somehow. But you are right, it isn't all that common. I much prefer ${\displaystyle G'}$, but [G, G] should be okay too, though it might look a bit cumbersome in the article.

## 'Smallest' normal subgp?

This article seems to claim a total order on normal subgps. This is probably a mistake but I wanted to check first.

Isaac (talk) 01:28, 19 December 2007 (UTC)

The article does not (intend to) claim that the normal subgroups are totally ordered, merely that a certain partially ordered subset of normal subgroups has a unique minimal element. The commutator subgroup is the intersection of all normal subgroups with abelian quotient and is itself such a normal subgroup, so it is the unique minimal element of the set of normal subgroups with abelian quotients, where the order is the partial order of set-wise inclusion. It is reasonably common in mathematics to use "smallest" for partial orders to be sort of a two-part assertion: (1) it is *a* minimal element, that is, there are no elements less than it in the partial order, and (2) it is the only minimal element, and even more, every element is greater than it in the partial order. A poset need not have a smallest element, but it can. Infimum or meet are other common words used in partial orders and lattices. The poset article uses the term smallest (with the sort of required qualifier, "if it exists"), and the lattice article uses "least" and "infimum" (where part of being a lattice is being a poset where smallest elements *do* exist; the lattice of normal subgroups is a lattice for instance). JackSchmidt (talk) 03:45, 19 December 2007 (UTC)

## Last line of Definition

"The commutator subgroup is a fully characteristic subgroup: it is closed under all endomorphisms of the group, which is stronger than normal."

Can someone clarify what is meant by the last clause 'which is stronger than normal'. Does it mean "The condition of being closed under all endomorphisms is a stronger condition than obtains for normal groups" ? As it is it kind of hangs in the air and isn't clear (to me anyway) Thx. Zero sharp (talk) 05:42, 10 April 2008 (UTC)

I've reworded it. --Zundark (talk) 07:45, 10 April 2008 (UTC)

## Some cleaning up

I made some changes in the article. Previously, the facts that the commutator subgroup was normal and moreover fully characteristic came up three times in the article, each time saying things slightly differently. I consolidated this information into one place farther up in the article. I also wrote out some simple "commutator identities" so as to make these properties as clear as possible. I noticed that there was no discussion of "commutators" per se so I added a section on that.

I reiterated the assertion of the two groups of order 96 in which the subset of commutators is not a subgroup, but in fact this was news to me (not that I disbelieve it): a reference is really needed here.

I also removed the passage about the (supposed) universal property of the commutator subgroup, together with its rather awkward footnote. I think this material was actually incorrect: a universal mapping property is something that defines an object up to canonical isomorphism in a certain category. The fact that it is only defined up to canonical isomorphism is an important feature of the definition. But there's no category here; we're just talking about the subgroups of a fixed group [true, you can view the lattice of subgroups as a category in which the morphisms are the inclusion maps, but that's a contrivance in this context] and indeed the commutator subgroup is truly unique, a clue that there is no [nontrivial] universal mapping property here.

On the other hand, the universal mapping property of the abelianization ought to be spelled out, although perhaps in a different article. Plclark (talk) 11:39, 3 July 2008 (UTC)Plclark

I noticed that JackSchmidt made some changes in my revisions. Thanks for this -- almost all the changes are clear improvements. Just a couple of thoughts:

1) This is picky, but I think that always -- and especially in the more formal context of an encylopedia, definitions should come before they are used, not after: so the conjugation notation should not be used and then defined followed by a "where..."

2) I had defined the commutator as ${\displaystyle ghg^{-1}h^{-1}}$ and this was changed back to having the inverses first. Of course there is no mathematical difference here. But in my experience, the convention I used seems more common and is a little more sensible (why throw in inverses? we could also define the commutator as ${\displaystyle hgh^{-1}g^{-1}}$ and that would be mathematically okay too, but shouldn't the most direct and transparent notation be used?). The article on commutators suggests that the "inverse first" convention is preferred by group theorists. Is this (still) true? I would be interested to hear from a group theorist on this.

Also, the section on abelianization should be expanded a bit, since it seems not to appear anywhere else on wikipedia. Does anyone else think that the use of terminology like "reflective subcategory" and "reflector" is more heavy-handed than necessary?

Thanks again -- it's a very positive experience to have one's improvements improved upon.

Plclark (talk) 21:52, 3 July 2008 (UTC)Plclark

Thanks for the explanations on the talk page. Today has been hectic, and I hadn't noticed the clear explanations here (luckily I don't think I messed up anything you explained). I definitely liked your reorganization. Looking at the diffs I was at first appalled you had deleted so much good material, but then when I looked at the article it was all still there! Magic? No just repetitive old version. The section on commutators seems like a very good idea. It has just the right amount of detail before sending the reader to commutator or down to the next section for the commutator subgroup proper.
I'll see if I can find a paper reference for the 96 thing. It is in a few textbooks (Dummit&Foote's Algebra, Isaacs's Character Theory, probably many others). Sadly it takes less than a second to search the small groups library in GAP, so it may not ever have been published as a "result". There was also some interesting thing about rearranging words and the relation to commutator subgroups and commutators, but I've forgotten the authors (I think it was a series of papers).
I'm never too fond of using categorical language to describe simple ideas in group theory. Even Glauberman's text on finite groups says that "conjugation functors" (a technical device that behaves well with conjugation and normalizers) are just called that, though you could make a category if you wanted.
So yes, I think if "reflective subcategory" is needed to understand a point in the article that is not intended for explaining category theory, then I think it is too heavy-handed. On the other hand, there is no reason not to have some explanations of category theory in this article, just like explaining a bit of the commutator subgroup in the commutator article. In other words, it is fine as "bonus material". I actually feel excluding these points of views (even if they are heavy handed) is a violation of WP:NPOV, but that policy also says they need only be given their due weight based on how prevalent that point of view is.
To answer your specific concerns about my edits: (1) I'm fine with moving the definition up. I believe it is done to your liking in the commutator article. I tend to think it is fine to have the definition in the same sentence. (2) This has always struck me as odd. More or less every group theorist uses gh = hg[g,h] = h g^h, the only exceptions being a few who came over from topology. However, really a significant portion of all other mathematicians (rarely use it at all, but when they do they) use gh = [g,h]hg. In the commutator article, both standards are discussed. In the commutator subgroup article basically it doesn't matter which way you do it, so I just try to keep it from changing.
So, I am fine with any fixes to 1 (sorry, I hadn't realized I broke it). I lean heavily towards keeping the "English" (and German) convention for gh=hg[g,h], but lean even more heavily towards trying not to mention it at all in this article. Definitely need more sources (in all the math articles!). You have my support for making all category theory stuff into "extras", rather than the main presentation, but try to keep some (correct) mention of it. JackSchmidt (talk) 22:38, 3 July 2008 (UTC)