Talk:Gδ set

quesstion

Would it perhaps make more sense to combine the $G_{\delta }$ and $F_{\sigma }$ articles into one article on Borel sets? --68.102.149.76 20:40, 16 September 2006 (UTC)[reply]

Seconded. The notions are surely strictly equivalent. Richard Pinch (talk) 18:34, 17 June 2008 (UTC)[reply]

To be clear, Gδ and Fσ are not synonyms, but they are very closely related (a subset of a topological space is Gδ if and only if its complement is Fσ). The rationals are Fσ but not Gδ. Borel sets are more general than both, but again in a very closely related way. Since Fσ set is so short, I think it might be reasonable to just merge it into this article. Since Borel set has two contradictory meanings, it might be best to kee those articles separate, but mention that Fσδσδσδ… is another word for (the first kind of) Borel set. JackSchmidt (talk) 20:42, 20 June 2008 (UTC)[reply]

Hrm, I think Trovatore made a reasonable point at Fσ set. At least it made the merge less obviously a good idea. Again with Fσ being so short, it seems easy to merge. JackSchmidt (talk) 20:51, 20 June 2008 (UTC)[reply]

If the articles got merged we'd have an article titled "F-sigma and G-delta sets" that would cover both each type individually as well as their dual relationship. Although in some respects F-sigma and G-delta sets are completely different, I think a merge would be best considering the size of the F-sigma article: its extremely small and doesn't look like it'll be expanded any time soon. --Blacklemon67 (talk) 01:36, 21 April 2015 (UTC)[reply]

That's an awkward title, though. Ordinarily a WP article should talk about a single thing named by its (usually grammatically singular) title. There are exceptions, but only when fitting it into that paradigm is especially difficult for some reason.

In this case, I don't see any special difficulty. Sure, the F-sigma article is short. So what? Let it be short. --Trovatore (talk) 02:11, 21 April 2015 (UTC)[reply]

I hear what you're saying. For what it's worth, I made a demo merge here to see how well the two topics inhabit the same article. I found myself finding a lot of correspondences in the examples and basic properties. In the end nearly everything in the F-sigma article reduced to "likewise"es and "vice-versa"s in the G-delta article. --Blacklemon67 (talk) 06:34, 21 April 2015 (UTC)[reply]

Continuous function

The fact that the set of points where aq function f is continuous is a G_δ set follows immediately from the fact that continuity at a point p can be defined by a $\Pi _{2}^{0}$ formula - the formula states that for every natural number E > 0 there exists a natural number N > 0 such that whenever $0<|x-p|<1/N$ , we have $|f(x)-f(p)|<1/E$ . If you fix a value of E, the set of x for which there is a corresponding N is an open set, and the universal quantifier on the E corresponds to the intersection of these sets. — Carl (CBM · talk) 12:15, 16 March 2008 (UTC)[reply]

Good point – I was wondering why this wasn’t in the article. I’ve added the above, wikifying some.

Thanks!

Nils von Barth (nbarth) (talk) 17:57, 31 August 2008 (UTC)[reply]

BTW, I believe that there is some converse (something like “Every G_δ can be realized as the points where some function is continuous”), but I forget the conditions – anyone know?

Nils von Barth (nbarth) (talk) 18:02, 31 August 2008 (UTC)[reply]

translation of Gebiet

Area is a misleading translation of Gebiet. I would say Gebiet means a geometric "domain", whereas "area" con be understood as the real number associated to it. —Preceding unsigned comment added by 78.129.56.66 (talk) 18:10, 9 January 2010 (UTC)[reply]

I agree that the translation of "domain" as "area" is misleading. From my experience, the German word "Gebiet" is usually used synonymously to the English word "neighborhood" in mathematics.-- Dr. scrubby-brush (talk) 01:13, 30 March 2010 (UTC)[reply]

Wait a minute — the article now claims that the G in G_δ is for Gebiet??? I thought it was for geöffnet. --Trovatore (talk) 21:20, 26 June 2010 (UTC)[reply]

The German word "Gebiet" means exactly what the English notion of "domain" means, namely an open connected set. A neighborhood is usually a domain (and therefore a Gebiet) but generally, one only speaks of "a neighborhood of some point or set", whereas a doman does not specify the location. I am not suer whether the G stands for Gebiet or geöffnet, but the translation of "Gebiet" neighborhood does not make much sense to me. Area might be possible, but as mentioned before, this is more likely to be the real number than the subset itself. Definitely "domain" is the best translation of Gebiet, see also the Wikipedia pages on Gebiet (Mathematik) and domain (mathematical analysis). As a reference, I am a German geometer who has studied both in the USA and in Germany, so I am familiar with both the German and the English terms. — Preceding unsigned comment added by Nemoline (talk • contribs) 21:34, 21 February 2013 (UTC)[reply]

Nowhere differentiable: attribution

The last example should rather cite Banach 1931, and Mazurkiewicz 1931, according to Real Analysis(2nd Edition), A.M. Bruckner, J.B. Bruckner, and B.S. Thomson, page 717. Boris Tsirelson (talk) 12:36, 14 November 2014 (UTC)[reply]

Reference for singletons in a T1 first countable space

Hi @David Eppstein. Every singleton in a fist countable T1 space is a G_delta set. The previous reference for this fact was https://math.stackexchange.com/questions/1882733/, which was not ideal because it was not a peer reviewed source. But at least it gave an explanation and some context for the fact. You replaced the reference with https://arxiv.org/abs/1104.2796 (or the peer reviewed equivalent). This has the benefit of having been peer reviewed. However, how can anyone think it is an improvement in this case? This paper is about deep issues at the frontier of research, and the fact in question is just stated in passing in the middle of a paragraph, without any indication about why it would be true. You and I know it's actually pretty simple to figure out, but to a casual reader, this new reference does not add any enlightenment to the particular fact. PatrickR2 (talk) 21:52, 30 June 2024 (UTC)[reply]

Neither does it subtract. It merely verifies the statement, in a way that meets Wikipedia requirements for sourcing. If readers want to look at the source and extract only that bare claim, keeping a blind eye to the broader issues it relates to, they can carry on doing so regardless. Also, "not ideal because it was not peer-reviewed" is kind of misleading. Stackexchange is not merely "not ideal"; it is forbidden because WP:USERGENERATED. For material at the level of an easy undergraduate exercise, I'm not convinced that it's necessary to find a textbook-level source that spells out the answer to the exercise, but we do still need a source that meets Wikipedia's standards for reliability. —David Eppstein (talk) 21:59, 30 June 2024 (UTC)[reply]

Found a better reference and replaced the previous one. Best of both worlds: a reliable textbook source, with context and explanation. PatrickR2 (talk) 20:20, 2 July 2024 (UTC)[reply]

Thanks, that is indeed an improvement. —David Eppstein (talk) 22:31, 2 July 2024 (UTC)[reply]