Talk:Norm (mathematics)

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Does anyone know what the modulus of an element of an arbitrary field is? Does the notion of modulus exist in every field?

You can't do it for all fields. The article as it stands right now is wrong. AxelBoldt 19:28, 22 Apr 2005 (UTC)

Contents 1 Link to an explanation of the || notation? 2 Merging norm (mathematics) and normed vector space 3 Any field? 4 Disambiguation??? 5 turning a seminorm into a norm 6 L2 and energy norms? 7 infinity norm 8 More about norms 9 Weak norms 10 Streamlined definition of a semi-norm 11 L_-inf

[edit] Link to an explanation of the || notation?

I don't know what the || notation means. From context, I think I get that it is analagous to absolute value or cardinality, but for vectors. Will someone please add something explictly introducing the || notation either here or on it's own page?

Kal Culus

I do not understand your question. || is a function on a vector space. We write |.| to indicate the function has to be feed a vector. We could also write f(x), with f the norm and x an element of the vector space V. The properties of the norm function are given in the article. I do not know how to make this more explicit in the article. MathMartin 13:33, 29 Oct 2004 (UTC)

[edit] Merging norm (mathematics) and normed vector space

Would it be agreeable to merge the material from here back into normed vector space? I can see a couple of reasons for this:

1. all examples for norms given here are also examples of normed vector spaces, so we could have all examples in one spot.
2. norms and normed vector spaces are essentially the same concept; in all likelihood, the reader who wants to learn about one wants to learn about the other as well.
3. the topology induced by a norm is currently explained in both articles
4. there are several other concepts called "norm" in mathematics, so norm (mathematics) should really be a disambiguation page. See norm.

AxelBoldt 19:59, 22 Apr 2005 (UTC)

I did not separate norm (mathematics) and normed vector space but I did separate metric (mathematics) and metric space recently and I think most of the arguments are valid for those pages as well. See Talk:Metric_space#Split_metric_space_into_metric_.28mathematics.29_and_metric_space

1. The examples should be separated better.
2. Although the concepts are closely related I think it is clearer to discuss them separately. The norm (mathematics) should focus in the norm as a special function on vector spaces whereas normed vector space should focus on the properties of a normed vector space.
3. The topology stuff should be discussed mainly on normed vector space
4. I have no problem with moving this article to norm (vector space) and putting a disambiguation page here instead.

MathMartin 20:53, 22 Apr 2005 (UTC)

If you are not convinced by my arguments feel free to merge the pages again. MathMartin 13:52, 23 Apr 2005 (UTC)

[edit] Any field?

This article defines norms for any field F, but there is a problem with axiom 2 since it relies on a real-valued function |a| defined in the field. What does that mean for a finite field, for example? --Zero 06:28, 23 Apr 2005 (UTC)

I really don't know whether it's possible to make the definition of norm general enough for more fields. I guess for the norm to exist, we need some kind of map from our base field to the reals which will itself satisfy the axioms of a norm. a sort of subadditive field homomorphism to R. Do such things exist for finite fields? I have no idea. Certainly I've never seen anything like that. -Lethe | Talk 17:25, July 11, 2005 (UTC)

[edit] Disambiguation???

On the Gauge disambiguation page, I tried to come up with a short one-line description, and wrote:

"*Gauge (mathematics), a semi norm, a concept related to convex sets.

Unfortunately, I don't understand this stuff well enough to know if I butchered the description or not. Could somebody please check it out and correct if needed? Thanks. RoySmith 8 July 2005 11:58 (UTC)

[edit] turning a seminorm into a norm

Somehow I seem to remember there being a person's name associated with the process of turning a seminorm into a norm by modding out by the vectors of norm zero, in just the same way that you turn the space of Lebesgue square integrable functions into a Banach space by identifying functions that agree almost everywhere. I came to wikipedia to look up this name, but couldn't find it. Am I imagining this? -Lethe | Talk 02:46, July 11, 2005 (UTC)

It works exactly like for Lebesgue case (which is in fact a special case of this): You just consider the quotient space modulo the subspace of vectors of zero seminorm, on which the (induced) seminorm (the seminorm is independent of the representative of a class) is a norm. (But I don't know whose name could be associated to this -- maybe you think of the associated Hausdorff space, where this is a special case of.) — MFH:Talk 21:59, 28 February 2006 (UTC)

For a finite field, I think there is only one. Define |x|=1 if x is not zero, and |0|=0. Not terribly useful! --Zero 17:02, 13 July 2005 (UTC)

[edit] L₂ and energy norms?

In my Finite Element Analysis course, I keep running into the L₂ norm,

and the "energy norm" (which apparently, for most solid mechanics applications, denotes energy):

Can someone who knows more than I on the subject add information on these? —BenFrantzDale 03:49, 27 September 2005 (UTC)

In the section p-norm in this article, there is a reference to Lp space where that kind of norms are discussed. Oleg Alexandrov 05:16, 27 September 2005 (UTC)

That article is not very well suited for explaining the L2 norm. IMO it's worth having a separate article, as it's used quite commonly. Info from Mathworld could be adapted. 142.103.235.1 19:30, 21 February 2006 (UTC)

You want L2 norm, then visit the article on L2 norm. :) Oleg Alexandrov (talk) 00:30, 22 February 2006 (UTC)

[edit] infinity norm

We need to generalize max to sup for infinity norm. MathStatWoman 09:26, 22 January 2006 (UTC)

I don't think that's right. If a sequence doesn't have a largest element, then its l-∞ norm is not finite. -lethe ^talk 10:24, 22 January 2006 (UTC)

[edit] More about norms

If we have an article entitled "Norm(mathematics)", should we not include the concepts of Banach spaces, norms based on the general Lebesgue integrals as well as countable sums, and sup norms? To quote from Banach spaces: "The Banach space l∞ consists of all bounded sequences of elements in K; the norm of such a sequence is defined to be the supremum of the absolute values of the sequence's members." MathStatWoman 13:04, 22 January 2006 (UTC)

• JA: Some of this material is distributed across the articles on Banach limits, Banach spaces, and Measure (mathematics), so maybe a better coordination and some expansion would do the trick. Jon Awbrey 15:32, 22 January 2006 (UTC)

If you want to add more stuff to this article, please go ahead. But don't do much integration work by removing stuff from other articles. That seldom works. Oleg Alexandrov (talk) 17:30, 22 January 2006 (UTC)

• JA: Yes, that's kinda what I meant by "co-ordination", interlinking and such. Jon Awbrey 17:33, 22 January 2006 (UTC)

We are on the same wavelenth. When I hear people taking big plans about reorganizing things I assume the worst. :) Happily not this time. :) Oleg Alexandrov (talk) 01:10, 23 January 2006 (UTC)

Someone should add some links (in this article) to other articles explaining norms, and the sup norm as an example, since it is so important in other areas of mathematics. Is that all right with everyone out there? MathStatWoman 18:33, 23 January 2006 (UTC)

Go ahead. :) Oleg Alexandrov (talk) 02:40, 24 January 2006 (UTC)

[edit] Weak norms

I found a weak norm being used at http://www.sm.luth.se/~johanb/applmath/chap3en/part3.htm. Should this be in the main article? --Sunnyside 10:40, 9 February 2006 (UTC)

I guess one needs to have a section in this article discussing norms on function spaces. That seems to be missing now. Then, that norm would be most welcome, but I never heard it being called weak norm though, some references for that (except the web page) would be needed. Oleg Alexandrov (talk) 23:02, 9 February 2006 (UTC)

[edit] Streamlined definition of a semi-norm

I prefer to define a semi-norm as in the latest version just posted today. Positive homogeneity and the triangle inequality are the only two properties you need, and it seems silly to keep carrying around positive definiteness when it follows. Perhaps I am missing something as I have not seen this anywhere. Jenny Harrison 12 July 2006

I reverted that change. It may be more streamlined but it is harder to read. I prefer the classical definition, and I added a brief sentence saying that first property is actually not necessary. Oleg Alexandrov (talk) 20:14, 12 July 2006 (UTC)

Let's think about this again. When we define a linear transformation T of a vector space, we do not usually list the fact that T(0) = 0 in the definition. We would appear ignorant if we did so. Similary, there is no need to include positive definitness. Since everyone has memorized it this way, most don't realize it is not needed. It becomes a mantra that may be comforting but is pointless to carry it around. The lean definition actually reads very well, for it looks a lot like the definition of a linear transformation, a kind of sublinear functional, as it were. So I am going to try again to make this read well for you. Jenny Harrison (talk) 16 July 2006 (UTC)

I see your point. However, T(0)=0 is trival to derive and not that essential for linear transformations. For seminorms, the positivity is crucial, and is important enough to actually state it as a definition. It is also rather clumsy to derive, and people better spend their attention on something else than that derivation when reading the article. So I disagree. Oleg Alexandrov (talk) 17:28, 17 July 2006 (UTC)

Hi, on a possibly related note, I noticed this under Notes:

A useful consequence of the norm axioms is the inequality ||u ± v|| ≥ | ||u|| − ||v|| | for all u and v ∈ K.

Would it be okay to move this along with positive definiteness as another property that follows from the definitions? The Absolute value article reads much better, witha section of fundamental properties and another for derived ones. MisterSheik 03:03, 17 July 2006 (UTC)

||u ± v|| ≥ | ||u|| − ||v|| | is indeed a derived property and followw very easily from the triangle inequality. Positivity is much more crucial and not as easy to derive and I think it should stay in the definition. Oleg Alexandrov (talk) 17:28, 17 July 2006 (UTC)

I still like the the way "absolute value" reads, and think that this article could benefit from immitating that model. My view is that all of the derived properties worth mentioning should be mentioned together. Also, I think that the positive definiteness is not that hard to derive :) (from 1, p(0)=p(0*u)=0*p(u)=0, and then let u=-v in 2, where p(-v)=p(v) from 1). MisterSheik 18:59, 17 July 2006 (UTC)

It is not worth the trouble spending time on the derivation, I think. Also, the absolute value article is different. There all the properties are consequences. Here are are talking about how to define an abstract concent like norm. Oleg Alexandrov (talk) 02:35, 18 July 2006 (UTC)

As MisterSheik points out, the derivation of positivity is rather trivial. The adjective "crucial" is subjective. All properties are crucial when you need them. The question comes down to this: what is fundamental and what is derived? If a derived property is included in a definition, then it must be verified each time one needs to check that a definition holds. It might be helpful to compare the definition of a linear transformation with the definition of a seminorm, with and without positivity. To show a linear transformation T is linear, we do not have to verify that T(0) = 0. Jenny Harrison (talk) 15:37, 18 July 2006 (UTC)

A similar discussion is going on at talk:metric space about the need for the positivity axiom. Oleg Alexandrov (talk) 00:50, 19 July 2006 (UTC)

[edit] L_-inf

Perhaps there is a L_(-infinity), with a unit circle of a cross (0 width)? —The preceding unsigned comment was added by 67.183.154.231 (talk) 04:01, 5 December 2006 (UTC).

what quite sure what you mean by inf there. if you consider the L^p norm on say R², with p < 1. this is not really a norm since the unit sphere is not convex. as p tends to 0, the unit circle probably looks more and more like a cross. Mct mht 09:03, 5 December 2006 (UTC)