# Talk:2 (number)

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WikiProject Mathematics (Rated C-class, High-importance)
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One of the 500 most frequently viewed mathematics articles.

## Arabic Glyph

isn't the origin of the arabic numbers related to the quantity of acute/right angles they have? 1 being the two sticks (without the bottom one), two was like a Z, 3 like an inverted E, and so on...? so this section of the article would seem a little bit dubious. Does this happen in other number articles? please revise. 159.90.161.23 19:50, 25 July 2006 (UTC)

Interesting, but very wrong. I'm counting four right (90 degree) angles in the inverted E.
Take a look at the Unicode code chart for Arabic. While you're at it, look for the book From one to zero: a universal history of numbers by Georges Ifrah, translated by Lowell Bair, New York : Viking, 1985. Anton Mravcek 21:53, 25 July 2006 (UTC)

## Couple

I think it's incorrect to redirect "Couple" to "Two". I was looking for the husband-wife usage here and didn't find it. Also "Two" is the wrong page to add the info. I guess Couple is more than a dictionary meaning, so it can have a page of its own. Jay 23:07, 20 Feb 2004 (UTC)

Thanks User:Icairns for removing the redirect and creating a disambiguation page. Jay 14:59, 1 Feb 2005 (UTC)

That did not really embetter the situation, though. I was looking for (gramatical) information about couple in the meaning of a relationship-group of (two) persons, and that it now disambiguates here-to is no better than redirecting here. I would change the dab page for couple, if only I knew where to it should point. The situation at pair is just the same, so pointing there makes no sense. Maybe someone else has an idea … Pardon my German (Fiiiisch!) (talk) 23:32, 29 July 2013 (UTC)

## $2 bill I removed the statement that "$2 bill "never gained widespread acceptance." It's nonsense. Such bills were in common circulation in the 1950s and 1960s, and cashiers customarily devoted a compartment of their cash register to them. They were less common than the $1,$5, and $10 bills, but by no means rare. And if memory serves, they were commoner at the time than, say, the silver dollar or the even the fifty-cent-piece, and far, far, far commoner than, say, either the Susan B. Anthony or gold-toned Sacagawea coins ever were. There was said to be a superstition that$2 bills were bad luck, but it was not a common superstition and neither I nor anyone I encountered had such a superstition. Dpbsmith 02:17, 14 Mar 2004 (UTC) P. S. It is, of course, very rare today since it was a form of U. S. currency known as the "United States Note" (red Treasury seal) which was vanishing even then and was discontinued circa 1966.

I have heard that at one time it was policy for the military to pay using $2 bills, which then would show up in local cash registers as an indication of the importance of the military to the local economy. If this was true at one time, it certainly was no longer true by the 79s, when I did my hitch. But my experience was that$2 were not in common circulation during the 50s or 60s; I had to go to a bank and specifically request one in order to be able to add one to my collection. Wschart (talk) 17:59, 22 May 2014 (UTC)
It shouldn't be in the article, regardless, even though all the comments seem to be in error. — Arthur Rubin (talk) 17:48, 29 May 2014 (UTC)

## Fancy picture

At 3 (telecommunications), there is a fancy picture of a 3. At Channel 4, there is a fancy picture of a 4. Do any Wikipedia articles mention a fancy picture of a 2?? Georgia guy 23:05, 18 Jan 2005 (UTC)

## Numerical prefixes for 2

Like the other numbers, 2 has a Greek numerical prefix and a Latin numerical prefix. Unlike the other numbers, however, there is also a prefix for 2, twi-, that is neither Greek nor Latin. What language is it?? Georgia guy 23:18, 20 Jan 2005 (UTC)

I'm not really sure it's a prefix - I would guess that "twice" and "twin" are just derived from "two", which itself comes from "duo". It's not like sticking "twi-" on the front of any other word would make sense. sjorford 15:01, 21 Jan 2005 (UTC)
My dictionary (Webster's NewWorld, college edition) says "twi-" is a prefix from Middle English via Old English. There are plenty of other twi- words; see my additions to twi-. dbenbenn | talk 16:04, 21 Jan 2005 (UTC)
I stand corrected...but twitch? twig? Do those really derive from "twi-" meaning "two"? Bizarre if so. sjorford 16:25, 21 Jan 2005 (UTC)
Good questions, now answered at twi-. dbenbenn | talk 20:37, 21 Jan 2005 (UTC)
And damn good answers, if I do say so my damn self. sjorford 21:53, 21 Jan 2005 (UTC)

## In astronomy

Messier object M2, a magnitude 6.5 globular cluster in the constellation Aquarius.
The New General Catalogue object NGC 2, a magnitude 14.2 spiral galaxy in the constellation Pegasus
The Saros number of the solar eclipse series which began on May 4 2861 BC and ended on June 21 1563 BC . The duration of Saros series 2 was 1298.1 years, and it contained 73 solar eclipses.
The Saros number of the lunar eclipse series which began on February 21 2541 BC and ended on April 22 1225 BC. The duration of Saros series 2 was 1316.2 years, and it contained 74 lunar eclipses.
These are all arbitrary number assignments. Nothing fundamental, like magic numbers and the like. -- Zanaq 18:29, 29 September 2005 (UTC)

Note that many pages on numbers, not only 2, have such astronomical data, and that the matter has already been discussed at Wikipedia:WikiProject Numbers (see for instance Wikipedia_talk:WikiProject_Numbers/Archive_1#Astronomical numbers). Note also that arbitrariness is not in itself a reason to exclude this information -- a lot of the stuff in sports, technology, etc. is just as arbitrary. So I've put the astronomical stuff back for consistency with the rest of the numbers, but you're welcome to bring this up at WikiProject Numbers. 4pq1injbok 22:00, 29 September 2005 (UTC)

I brought it up. -- Zanaq 11:28, 1 October 2005 (UTC)

## Redirected from 'too'

I was redirected to this page from the search term 'too'. Shouldn't this redirect be revised, considering the meaning of 'too' is completely different to 'two'?

## Redirected from '1 + 1'

I think 1 + 1 should have an independent account, given that 1 + 1 is a fundamental component of mathematics, logic, language and popular culture, with connotations that go far beyond the simple fact that it '= 2'. To automatically redirect to 2 is somewhat of an oversight. Thefuguestate 20:23, 5 December 2006 (UTC)

Perhaps it should exist as an article. However, it shouldn't exist as a redirect to this article as Wikipedia is not a calculator. Astroguy2 10:00, 5 June 2007 (UTC)
True; and that's why I fixed the redirect from the square root of 4 to here, too. Dicklyon 06:56, 12 August 2007 (UTC)

## Merge with square root of 4

Please discuss this in the Talk:Square root of 4 since the latter has been nominated for deletion. I think there is some interesting material in the latter page (Computer Science stuff, about the error which, if from real reliable sources, might merit mentioning) that can be added here. Furthermore, the additional material will enrich this page. Finally, mathematical views can also be discussed. Brusegadi 06:07, 12 August 2007 (UTC)

Also worth pointing out that many edits have been made by a user to prove a point. I suggest neglecting that part of the page. Brusegadi 06:50, 12 August 2007 (UTC)
The "computer science" bit was actually a bit of a joke, since the point of the article is unrelated to the particular value that he used as his title example. Just shows that not everything that mentions a number is a reliable source. Dicklyon 06:55, 12 August 2007 (UTC)
The whole thing that computers 'do not think so' made a little sense to me. I thought it would be one of those articles with strange titles. I thought you had access to the link so I took it seriously; I did not go over it because I thought it would be a waste of time; I wanted to wait and see what others had to say. To be honest the whole 2.00000... irritated me (I am a bit tired and using this to procrastinate on long ago due work). It was a good wiki day thought. I never knew that so much stuff could be done to prove a point.  ;) Brusegadi 08:38, 12 August 2007 (UTC)
The article you mention was real and serious, but not really relevant to the square root of 4 (it's been a long time since there were computers that would get square roots of square integers wrong). If you want to look for any bits worth salvaging, or just want to enjoy the parody on the square root of 5, find it at User:dicklyon/Square_root_of_4. Dicklyon 15:29, 12 August 2007 (UTC)

## Citation Needed?

From the article: "2 (two) is a number, numeral, and glyph. It is the natural number following 1 and preceding 3.[citation needed]" I just want to say how glad I am to see the citation needed thingy at the end of this sentence. I just don't feel comfortable believing this information until I know it comes from a credible source. I mean, anyone could have posted this, and I don't know if they have a degree in math, are merely numbers enthusiasts, or are 5 years old. Thanks wikipediers. --Banyan 02:28, 8 October 2007 (UTC)

Yes, and it gives me a good idea about other articles where more citations may be needed... Dicklyon 02:38, 8 October 2007 (UTC)
Sarcasm in teh wrong hands is just plain stupid.[citation needed] Numerao (talk) 22:18, 8 March 2008 (UTC)

## Brace (sports) redirect?

Brace (sports) redirects here. Please correct or put in some indication as to why. 99.11.160.111 (talk) 21:51, 17 December 2009 (UTC) -- brace, "a pair; two, esp of game birds: a brace of partridges".. —Preceding unsigned comment added by 86.161.154.28 (talk) 07:58, 23 October 2010 (UTC)

## Full width symbol

Does anyone know the difference between the symbols 2 and ２? I understand that the last one is a "full-width 2" used in Japanese, according to Wiktionary (http://en.wiktionary.org/wiki/%EF%BC%92). Can anyone say why they use that instead of the regular 2 symbol? 77.57.214.157 (talk) 09:15, 29 December 2009 (UTC) (lKj)

## Organization of number pages and number disambiguation pages

Dear Colleagues,

There is an ongoing discussion on the organization of number pages and number disambiguation pages.

Cheers,

PolarYukon (talk) 15:28, 8 January 2010 (UTC)

## 2 is the only even prime?

Well of course it is; the definition of even is integral divisibility by 2. Every integer (prime or not) is integrally divisible by itself, it goes without saying. Saying that 2 is the only even prime is really more a attribute of human language, specifically that we have a word that means integrally divisible by 2. Although we don't have a word that means integrally divisible by 3, 3 is the only prime that is thricive, 5 is the only prime that is pentic, 7 is the only prime that is septic, yada yada yada. If humans had never existed 2 would still be prime and divisible by itself just as much as 3 would, but also they would equally not have a word describing those attributes.

So what? So this should be noted. Refer to Three where Wikipedia says that 3 is the only prime that is 1 less than a perfect square, an explanation follows that describes how (n-1)=1 and (n+1)=3 for n2-1. I think an analogous explanation of 2 being the only even prime is warranted since this is similarly not so magically so as people may be inclined to think upon learning this datum. (and its literally infinitely more work editing all the other prime pages to say that the other primes are the only prime divisible by themselves.) Grabba (talk) 07:13, 29 January 2010 (UTC)

## Triangular Connection

I'm removing the following text:

Two has a connection to triangular numbers:

${\displaystyle \prod _{k=0}^{n}2^{k}=2^{tri_{2}(n)}}$

where ${\displaystyle tri_{d}(n)={\frac {1}{d!}}\prod _{k=0}^{d-1}(n+k)\quad {\mbox{if}}\quad d\geq 2}$ gives the nth d-dimensional simplex number. When d=2,

${\displaystyle tri_{2}(n)={\frac {n^{2}+n}{2}}.}$

because it's not unique for 2. Actually any number Q has the property ${\displaystyle \prod _{k=0}^{n}Q^{k}=Q^{tri_{2}(n)}}$, because it is a product of common bases. A lot of the other examples are so silly it's hard to actually take this seriously. It seems like most of the mathematics formulas examples are trying to say "2 is the only solution to this equation, and this is a significant equation, and it is significant that 2 is the only solution". I don't think many of them meet that requirement. Antares5245 (talk) 12:31, 10 July 2010 (UTC)

## Tetration question

"Two also has the unique property that 2+2 = 2·2 = 2²=2↑↑2=2↑↑↑2, and so on, no matter how high the operation is." Is this really so? My understanding is that 2↑↑↑2 = 2↑↑2↑↑2 = 2↑↑(2²) = 2↑↑4 = 65536. Did I miss something? Marco 18:34, 20 July 2010 (UTC)

Yes. At the step 2↑↑↑2 = 2↑↑2↑↑2, I think you missed this: According to the Knuth's up-arrow notation article,
${\displaystyle {\begin{matrix}a\uparrow \uparrow \uparrow b=&\underbrace {a_{}\uparrow \uparrow (a\uparrow \uparrow (\dots \uparrow \uparrow a))} \\&b{\mbox{ copies of }}a\end{matrix}}}$
therefore 2↑↑↑2 = 2↑↑2 (2 copies of 2), not 2↑↑2↑↑2 (3 copies of 2). Art LaPella (talk) 17:59, 6 October 2011 (UTC)

## Duality

The number two is spelled out everywhere in life: Man/Woman, Day/Night, -infinity/+infinity, binary stars, good/evil, hot/cold, birth/death, etc... I was also reading into some Quantum physics articles, and I read something somewhere talking about "either/or" in relation to some theory, and I couldn't help but think about this duality property. Anybody else feel this way? Ulterion (talk) 06:01, 16 August 2010 (UTC)

## New subtitle

I want to see a new subtitle on the top that says:

II and Two redirect here. For other uses, see II (disambiguation) and Two (disambiguation)

Leave some notes here. Lamp301 (talk) 02:45, 9 January 2011 (UTC)

## Origin of "two" (spelling), & "twain" disambiguation

I would like the article on "two" to describe the origin, history and etymology of its spelling and pronunciation, which isn't in the article. (The article on "forty" somewhat describes why it is spelled as "forty" instead of "fourty", so I think "two" should, too -- no pun intended.)

Why is the "w" in "two" silent? Where does the written word "two" come from? I believe "two", "twin", "twice", and "twain" have the same root word. (Is "twilight" related?)

I think the article on "Mark Twain" should have a link to the disambiguation page for "twain" and "two", like the "TWAIN" article at en.wikipedia.org/wiki/TWAIN does.

I agree. I came here looking for information about the word, and I don't find it here are in 2 (disambiguation). --Tysto (talk) 07:03, 8 May 2012 (UTC)

## maybe add this fact ?

2 is the only real number such that

${\displaystyle a={\sqrt {a+{\sqrt {a+{\sqrt {a+{\sqrt {\ldots }}}}}}}}}$ — Preceding unsigned comment added by 178.7.109.249 (talk) 14:37, 9 June 2012 (UTC)
True. Notable? Anyway, that's not the way you wrote it. — Arthur Rubin (talk) 16:37, 9 June 2012 (UTC)

Well if you like this one better.

${\displaystyle 2={\sqrt {2+{\sqrt {2+{\sqrt {2+{\sqrt {\ldots }}}}}}}}}$

if this is not notable then the statement about the geometric sum isnt either. — Preceding unsigned comment added by 178.7.109.249 (talk) 21:29, 9 June 2012 (UTC)

## Move discussion in progress

There is a move discussion in progress on Talk:1 (number) which affects this page. Please participate on that page and not in this talk page section. Thank you. —RMCD bot 00:43, 18 July 2013 (UTC)

## "Unique" Mathematical Property

In the "In mathematics" section, it is noted that 2 is the only number b for which ${\displaystyle \sum _{k=0}^{n-1}b^{k}=b^{n}-1}$ is true for every positive integer n. However, that's not really anything special about the number 2. ${\displaystyle \sum _{k=0}^{n-1}b^{k}\times (b-1)=b^{n}-1}$ is true for all combinations of positive integers b and n, so it's clear that this "unique" property of 2 is a result of the fact that 2 - 1 = 1.

(How) Should the article be changed to reflect this? — Preceding unsigned comment added by Orthoplex64 (talkcontribs) 15:38, 17 May 2014 (UTC)

## SS0

I am puzzled by the recent addition of the redirection of SS0 to 2_(number). ("SS0" redirects here.) What is "SS0" and why is it redirecting here? Dhrm77 (talk) 03:33, 21 February 2016 (UTC)

It is the representation of the number 2 in Peano arithmetic, as it is the successor of the successor of 0: but S(S(0)) would be more proper. A bit silly if you ask me. Double sharp (talk) 14:13, 18 April 2016 (UTC)

## Assessment comment

The comment(s) below were originally left at Talk:2 (number)/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.

 The section entitled In mathematics appears to consist of a haphazard collection of properties, many of which are not notable.  --LambiamTalk 09:05, 3 June 2007 (UTC)

Last edited at 09:05, 3 June 2007 (UTC). Substituted at 19:43, 1 May 2016 (UTC)