Talk:Real analysis

Initial comments[edit]

Isn't this suficiently dealt with on the mathematical analysis page? Why is a separate page needed? -Stuart

This page's structure - explaining what sequence a book would introduce the subtopics - doesn't really make an encyclopedia entry i don't think... Enochlau 17:29, 21 Apr 2005 (UTC)

Old page - we do things differently now. Charles Matthews 17:38, 21 Apr 2005 (UTC)

Yes this page needs to be rewritten ;-) Paul August ☎ 17:54, Apr 21, 2005 (UTC)

I would like to see some examples of where this sort of extremely abstract mathematics is used in the field, particularly with links to the appropriate astrophysics and quantum theory pages. -Eliezer Kanal 11:24 PM, Oct 15, 2005

Well, I do think examples would be nice, but real analysis is not just used in those fields, and it can be very limiting to say "this is what it's for". Analysis is a stepping stone to a lot of other mathematics. Higher level study of probability that's used in mathematical finance requires analysis, for example. I think some of the "real-life usages" in other articles have made those articles worse. Tristanreid 19:20, 16 October 2005 (UTC)[reply]

I just wasn't sure if it was proper to state that the Real's are constructed by Cauchy sequences, when the basis for their definition relies on the completeness axiom and concept of supremum and infimum. If there is another approach to defining a Cauchy sequence I appologize.

I would say that the least upper bound property is far and away the most important property of the real numbers, not the properties of the absolute value function as stated in this article.

I added a little bit based on the reference which I also added - I just took my Intro to Real Analysis final on Friday :) If I've done an improper edit, please forgive me because I have only just begun trying to contribute. Anastas5425 21:04, 23 April 2007 (UTC)[reply]

of course real numbers can be defined as an ordered field of characteristic 0, that has least upperbound property. so least upperbound property is of course the most important. Jeroje 20:37, 14 July 2007 (UTC)jeroje[reply]

This article is appallingly bad...I don't think it's already covered in mathematical analysis since that article (as it should) points the reader to a whole big list of specific sub-fields of mathematical analysis. This article should look more like non-standard analysis, complex analysis, or at least functional analysis. A good start would be, say, anything that doesn't look like a course outline. Specific techniques like epsilon-delta proofs should be mentioned, a history section should exist, and lots of other stuff - I'll be starting on it tomorrow. Meowist 11:44, 21 June 2007 (UTC)[reply]

As I mention below, I've redone major parts of the article, and I've tried to accomodate as many of the comments on here as possible, but there are a couple of things Meowist suggested that I haven't done, and I thought I should explain why.

Writing an article on real analysis is a bit like writing an article on non-hybrid cars. For a long time, all analysis was real analysis, but recently other techniques have emerged, and it became meaningful to talk about real analysis as being different to complex or functional analysis. You can't really talk about the history of real analysis as being distinct from the history of analysis as a whole, any more than you can separate the history of the car from the non-hybrid car.

Also, including a section about epsilon-delta proofs seemed a little inappropriate, as this is a technique that is not specific to real analysis, but is common to all areas of analysis. It would be like putting a section on headlamps in the hypothetical article on non-hybrid cars.

Anyway, I hope that explains some of my editorial choices, and I apologise for the lousy metaphor. James pic 13:19, 26 June 2007 (UTC)[reply]

I am wondering, what is the purpose of having this page with not even an introduction of what it really is? lucarioic 03:50 19 Oct 2012 (UTC)

Overhaul[edit]

I've taken the liberty of redoing large parts of this article. Much of the material was about analysis in general, and not specific to real analysis, and a large portion of it simply outlined the syllabus of a first course in real analysis. I've tried as best I can to keep the stuff that's relevant, to remove the stuff that's better dealt with elsewhere (or just plain unsuitable), and to put the material into context, although obviously more work will be needed. This article was an embarassment, and I can't claim to have fixed it, but I've tried to clean it up, and make it more maintainable. Please edit ruthlessly. James pic 12:09, 26 June 2007 (UTC)[reply]

I am very pleased with your rewrite. In response to your comments, in hindsight, a history section would perhaps mirror too closely a proper history section in Mathematical Analysis (the one there is hideous). If analysis is likened to a tree, then real analysis is the stem from which everything else forks. What I had in mind for the section with epsilon-delta proofs was simply an example of analysis concepts being used in a proof of real-analyis-type statement - nothing like an actually famous theorem, just some small slightly non-obvious claim. Here's an example of what I'd put in: Proving the limit of \sqrt(n^2+n)-n as n goes to infinity is 1/2. I'd use the continuity of sqrt(), and some epsilon-delta. What do you think about the general idea of including an example or/and this one in particular? Meowist 02:29, 27 June 2007 (UTC)[reply]

Certainly, an example like the one you suggested could be useful. As for choice of example, I'd probably go with something that demonstrates the character of real analysis, such as the monotone convergence theorem - the proof is a simple application of the least upper bound property and some simple delta-epsilonics. Obviously though, it's not my article; make whatever changes you feel are appropriate. James pic 12:54, 27 June 2007 (UTC)[reply]

Correct redirect?[edit]

Real function redirects here. Is this right? I was actually looking for the article Real part when I got here. My english skills are too limited in this area, but it seemed like a reasonable search string for that article. Is there any point in adding some kind of see also... or for other uses... section? -- Asked by... 83.227.131.25 (talk) 13:31, 31 March 2009 (UTC)[reply]

I've never heard that use of "real function", but I can see the logic behind it.-Wafulz (talk) 15:09, 31 March 2009 (UTC)[reply]

Thanks for your answer. If it doesn't make sence to native english speakers, forget it – it was probably just a matter of bad translation or misunderstanding from my side. I've also seen it written as

\operatorname {Re} (z)

, which kind of looks like a function. Cheers, says 83.227.131.25 (talk) 15:54, 31 March 2009 (UTC)[reply]

Set or field of real numbers?[edit]

An editor changed the description of real analysis from dealing with the "set" of real numbers to the "field" of real numbers. I question whether this is an improvement. Of course, I do not deny that the real numbers form an algebraic field, but the topic of real analysis is not specifically algebraic. One might as well say that it deals with the "space" of real numbers. What do you folks think? TomS TDotO (talk) 09:21, 15 August 2010 (UTC)[reply]

Does the word "set" even need to be in their? "Real analysis, or theory of functions of a real variable is a branch of mathematical analysis dealing with the real numbers" — Preceding unsigned comment added by Brad7777 (talk • contribs) 15:49, 23 October 2011 (UTC)[reply]

@(TomS TDotO): Real analysis is not algebraic? Let's see, with the set of real numbers we can talk about unions and intersections, and that's about all (and incidentally, we can't tell them apart from the set of complex numbers). If we want to do some fancy stuff like add or multiply them, then we've got to consider the field of real numbers. On the other hand, I still agree with you that just because it's true, doesn't mean we have to say it in the lead, and I'm going to implement Brad's suggestion to just call them "the real numbers". — ChalkboardCowboy^[T] 20:20, 6 July 2012 (UTC)[reply]

I like the idea of field because it emphasizes more structure and also provides a relationship to other branches of mathematics. But perhaps it would be more accessible or simple to just say that it deals with the "real numbers". Cazort (talk) 17:39, 18 October 2012 (UTC)[reply]

Proposed merge[edit]

In analogy with function of a complex variable and complex-valued function redirecting to complex analysis, it seems natural that the two stub articles function of a real variable and real-valued function should be merged to this article. Comments? Isheden (talk) 17:47, 30 July 2012 (UTC)[reply]

This seems like a problem with the complex analysis article: if I didn't know what a complex-valued function was I don't think I would be able to figure it out from the complex analysis article. Same thing with function of a complex variable. There is a little that can be said in these other articles that probably shouldn't be in this article, so I think they should stay separate. Jbeyerl (talk) 12:50, 1 September 2012 (UTC)[reply]

I don't like the idea of merging and redirecting any of these things. Complex analysis and real analysis are fields, and complex-valued functions are objects of study in these fields. Both seem notable as subjects in and of their own. Cazort (talk) 17:33, 18 October 2012 (UTC)[reply]

Agree with the merge --- function of a real variable and real-valued function are definitions only and trivial definitions. We can't have them as stubs. Also, when they are objects of study, we are talking about Real analysis or Mathematical analysis.

Thanks, Anand ^{(talk page)} 02:38, 7 May 2013 (UTC)[reply]

expert-subject tag?[edit]

This article was tagged as needing attention from an expert, with no specific reason expressed. I read the article and didn't see anything glaringly inaccurate. What was the issue that needed to be addressed? Jbeyerl (talk) 12:56, 1 September 2012 (UTC)[reply]

"The foundation of real analysis"?[edit]

At the Key concepts section, the article claims that "[t]he foundation of real analysis is the construction of the real numbers from the rational numbers." I understand this pedagogically, but might this be potentially misleading? My understanding is that what enables one to do useful analysis is that the reals form an ordered field that's complete (both in the sense of Cauchy-completeness and Dedekind-completeness). Indeed, from the page Real_number, up to field-isomorphism, the reals are unique as the only totally ordered field, complete and Archimedean with respect to that ordering.

From this view, it's this field- and order-structure of $\mathbb {R}$ that is central. The importance of the constructions of $\mathbb {R}$ from $\mathbb {Q}$ (via Dedekind cuts or equivalence classes of Cauchy sequences of rationals or nested intervals with rational endpoints or whatever) is that it concretely demonstrates the existence of an object with these desired properties. (Subject, of course, to acceptance of the logical consistency of the rationals in the first place.) There are, however, other ways of constructing $\mathbb {R}$ .

For example, in Numbers by Ebbinghaus et al., Springer-Verlag, 1991, ISBN 0-387-97497-0 (itself a translation of the second edition of the German Zahlen, 1988, Springer Verlag), Section 2.5 describes an axiomatic definition of the reals. (See also Construction_of_the_real_numbers#Synthetic_approach and Tarski's_axiomatization_of_the_reals.) This vantage point views the reals ab ovo, and we then show that we can recover $\mathbb {N} \subset \mathbb {Z} \subset \mathbb {Q} \subset \mathbb {R}$ with their usual respective properties. In Chapter 13 (Ibid.), the author outlines John Conway's "game" method for constructing $\mathbb {R}$ "in a single step."

I would provisionally propose alternate wording like the following, borrowing from Real_number:

The foundation of real analysis is the existence of a Archimedean complete totally ordered field

(\mathbb {R} ,+,\cdot ,<)

, unique up to isomorphism of ordered fields. The standard method of verifying the existence of such a structure is constructing the real numbers from the rational numbers,

\mathbb {Q}

. Uniqueness typically follows from a separate argument.

I am open to suggestions about whether the original wording merits changing in the first place, whether the above proposed wording would indeed constitute an improvement, and what other changes might further improve this section.

As an independent matter, Numbers cited above may prove useful to authors who wish to expand this article, especially for content regarding the historical development of real analysis. — Preceding unsigned comment added by 71.65.211.107 (talk) 07:12, 14 October 2012 (UTC)[reply]

I think it is subjective to claim that any one approach is "the foundation" of a subject. I think it's more fitting with WP:NPOV to write things like: "The subject of real analysis can be developed from the construction of the real numbers from the rational numbers." But this doesn't reflect the historical development of the subject--many results came before the construction was carried out. And one can work in the subject without ever constructing the real numbers, accepting them as a given. I also want to point out that mathematical constructivists, rejecting the axiom of choice would have a different perspective--yet there is still that can be done within the field of real analysis in a constructivist framework. It's important that we don't present a single POV as the only one, or give undue weight to one. Cazort (talk) 17:36, 18 October 2012 (UTC)[reply]

I agree that "foundation" is probably a problematic word here, whether as in the original or in the rewording. Does this mean logical foundation, historical foundation, some hybrid between the two, or something else entirely?

That said, if there is to be a "Key Concepts" section, I think it's defensible to on neutrality grounds to emphasize those models of

\mathbb {R}

which lead to the subsequently-enumerated useful, familiar theorems most likely to be accessible to a lay audience: Bolzano-Weierstrass, Heine-Borel, intermediate value theorem, etc.

I'm not sufficiently knowledgeable about constructivist models to know which theorems, if any, might break down in constructivist or other cases. But if detailing all the various models or methods for constructing

\mathbb {R}

would be a digression for the sake of NPOV, then might it simply be more sensible to append constructing the reals to the See also section and remove this sentence entirely? Perhaps a wholesale reorganization of this section would be merited. 71.65.211.107 (talk) 17:57, 19 October 2012 (UTC)[reply]

General Metric Spaces[edit]

In some parts of this page, for example the definition of Uniform Continuity, definitions are constructed in the setting of general metric spaces (X,d). Given that this article is purely about the real analysis, would it not make more sense to define everything less abstractly, in terms of the real numbers rather than a general set X? Someone looking for information on abstract analysis would surely go the the page Mathematical Analysis or, in this specific case, Uniform Continuity. EdwardRussell (talk) 17:13, 11 November 2013 (UTC)[reply]

I deleted the metric space material; I added it in accidentally when I wrote that section. Thanks! Brirush (talk) 20:27, 11 November 2013 (UTC)[reply]

Assessment comment[edit]

The comment(s) below were originally left at Talk:Real analysis/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

Solid block of text needs breaking up into sections, with information on history, motivation, applications and examples. Tompw 17:43, 7 October 2006 (UTC)[reply]

Last edited at 12:06, 16 April 2007 (UTC). Substituted at 02:32, 5 May 2016 (UTC)

Compactness[edit]

To D.Lazard, regarding the recent revert, I justify the need for a compactness section because a careful reading reveals that the term compactness is used for the definition of a term in the section 'absolute continuity', as well as the statement that continuous functions are uniformly continuous on compact sets. It is unjustifiable not to define such an important real analysis concept. As for the charge that the section is irrelevant, I point to the focus on the closed and bounded definition as the most relevant for real analysis.

No first semester course in real analysis can neglect compactness as a topic, so in my opinion it must be included. I authored many of the definitions on this page, but was surprised to find that compactness had not be defined.

I thank you for your attention to this article. Your revert is reverted.

Alsosaid1987 (talk) 20:15, 5 May 2018 (UTC)[reply]

Before you revert, also pay attention to the changes in the construction of real numbers section. Before, a statement seemed to imply that order by < is purely an algebraic feature of the reals, which is unreasonable. Alsosaid1987 (talk) 20:25, 5 May 2018 (UTC)[reply]

Please read WP:BRD: When one of your edits is reverted, you must not revert it again for starting WP:edit warring. Instead, you must start a discussion on the talk page in view of a WP:consensus. So, I will revert your revert until we get other editors opinions resulting in a consensus.

About the section "Compactness", as said in my edit summary, I agree that such a section is relevant here. But this makes no sense to repeat here the article Compact space (Wikipedia is not a textbook, but an encyclopedia). On the other hand a section on compactness in

\mathbb {R}

and

\mathbb {R} ^{n}

would be useful.

About your other edits, some are useful but some not. Typically, you introduce many concepts that are related to real analysis but that are not interesting for most readers that are specifically interested to the subject of the article. The way of mentioning these concepts is also too technical for most readers (think to engineers that are concerned with real analysis in their work). This is the case of your way of presenting compactness, and also the introduction of general topology and metric spaces in the section on the construction of real numbers. There are probably other examples, but I have not yet got the time for reviewing all your edits. I will thus do other reverts. D.Lazard (talk) 09:02, 6 May 2018 (UTC)[reply]

User:D.Lazard, thanks for the well-considered changes. I will make the section on compactness shorter, if that would address your criticism that it's a repeat of the article. Alsosaid1987 (talk) 15:29, 6 May 2018 (UTC)[reply]

Although the previous location might have been inappropriate, I think there needs to be some mention that R is a metric space. For now, I will add a new section, but feel free to move it where you feel is appropriate. Alsosaid1987 (talk) 16:31, 6 May 2018 (UTC)[reply]

A summary of changes: More R-specific things in the compactness section, including examples and non-examples in R. Removed reference to compactness as topological property. Shortened definition of cover and subcover. Started a new short section on the topological properties of R, where R as the prototypical metric space is mentioned.

Please evaluate whether there are better ways to do these things. Alsosaid1987 (talk) 17:07, 6 May 2018 (UTC)[reply]

Scope of the article[edit]

This post is somehow a reply to the last post of the preceding section. This article suffers of multiples issues.

The lead says that it is about real functions, that is real-valued functions of a real variables, and this is almost nothing in it that is about real functions. Note that Real function redirects here, and despite the importance of this subject, it is not specifically covered in any other Wikipedia article.
The body of the article duplicates Calculus and, partly, Mathematical analysis, and a reader has no mean to know where searching the information that he needs.
The article does not respect MOS:MATH
It contains many errors or confusing formulations: In particularity it considers systematically functions that are defined on arbitrary subsets of the reals, while real analysis is specifically the study of functions whose domain contains an interval of positive length.

Therefore, in my opinion, the article must be completely rewritten for covering its subject, are defined in the lead, and nothing else. Some thoughts? D.Lazard (talk) 18:03, 6 May 2018 (UTC)[reply]

User:D.Lazard, I must disagree that real analysis is about functions on intervals. In my opinion, functions on any subset of the reals are well within the purview of real analysis. Arguably, functions on any subset of R^n are as well. After all, functions f:N->R (i.e., sequences) are studied as part of real analysis!

However, I can see that the emphasis should probably be on [a,b] or on R, so I think it's reasonable to remove or de-emphasize some of the more general definitions, with a short note that there are more general definitions, to be consulted on another page. I think that might also help with my perception that the article has become a bit long. I acknowledge my own contributions to this problem.

As for MOS:MATH, could you point to any stipulation or section in particular?

Are you aware of the article Function of a real variable? It seems like this might be a problem with a bad redirect. Alsosaid1987 (talk) 18:25, 6 May 2018 (UTC)[reply]

This article is about vector-valued functions. D.Lazard (talk) 19:47, 6 May 2018 (UTC)[reply]

I think the information you want presented is split between that article, in which the domain is R, and this one, Real-valued function, in which the codomain is R. It is unfortunate that they are split into two pages, so if you want to add f:R->R-specific material to the real analysis article, I support it. However, to construe real analysis as only about f:R->R is too narrow, in my opinion. 67.186.58.77 (talk) 23:14, 6 May 2018 (UTC) That was me. Alsosaid1987 (talk) 23:15, 6 May 2018 (UTC)[reply]

Per MOS:MATH recommendations, I have created a new last section on generalizations that real analysis had given rise to, with ample links to other concepts and disciplines of mathematics. I'm not sure if the current format is okay, or a bullet-point format would be clearer. Previously, these generalizations formed a paragraph in the important results section, which does not seem to be the most appropriate venue. Alsosaid1987 (talk) 04:20, 7 May 2018 (UTC)[reply]

I have not yet reviewed your edit, and thus I'll not comment them. Nevertheless I agree that real analysis cannot be reduced to the study of the real-valued functions of a real variable. This means that Real function must be redirected elsewhere or be made a true article. This means that the lead of Real analysis must be rewritten for reflecting the content of the article and clarifying its scope. IMO, it is only when this will be done that one could decide if your additions to the article really belong to it.

About real functions, it is amazing that I cannot find a single Wikipedia article that collect their basic properties and the basic methods for studying the properties of a given real function (what is taught or should be taught in every first level course of calculus). For filling this gap, I'll start by redirecting Real function to Function of a real variable, and editing this article. It is possible that this will lead to split this article, but this must be decided later. D.Lazard (talk) 09:20, 9 May 2018 (UTC)[reply]

I support your change to the redirect. I think to fix the lead of Real analysis, we need to first an authoritative (or consensus) definition of what real analysis is or studies. Unfortunately, the well-known undergrad texts of Rudin, Apostol, and Pugh that I'm familiar with define their subject. For what it's worth, the Wikibook on real analysis says "The subject of real analysis is concerned with studying the behavior and properties of functions, sequences, and sets on the real number line." Alsosaid1987 (talk) 06:45, 10 May 2018 (UTC)[reply]

After much searching and comparison, I went with Terence Tao's definition of real analysis. It feels somewhat piecemeal and ad hoc, but I feel like it's complete. Other possibilities for a definition should be suggested and discussed. Alsosaid1987 (talk) 05:47, 12 May 2018 (UTC)[reply]

not written for general user[edit]

this is like a parody of an article written for the general user if your mom or dad can't get it , it is at to high a level, at least for the intro and first few graphs I am so tired of you math people and your arrogance and your inability or refusal to write things at the right level signed, a guy who has spent a lot of time on biotech wiki — Preceding unsigned comment added by 50.245.17.105 (talk) 21:03, 4 August 2020 (UTC)[reply]

On the cleanup tag at Scope[edit]

Should it be removed, or is it appropriate/convenient/necessary to have a discussion on the accessibility level of this article, as the comment above me seems to believe? Horsesizedduck (talk) 16:22, 8 June 2021 (UTC)[reply]

The previous post is clearly written by someone that clearly does not know anything about mathematics, and hate them. So it needs not to be considered further.

The tag clearly about the section, not the whole article. I edited it for clarity.

Please, do not remove the tag, as I essentially agree with its rationale. IMO, this does not require a discussion. This requires simply some time to spent by an editor with some competence in writing Wikipedia mathematical articles, an a minimal competence in the subject of the article. D.Lazard (talk) 17:40, 8 June 2021 (UTC)[reply]